Mongole soup (called “Purée Mongole” in the earliest descriptions, but also spelled as “Purée Mongol,” “Potage Mongole,” “Potage Mongol,” “Creme Mongole” and “Creme Mongol”) was popular in New York City in the 1930s, being served at several Manhattan restaurants. Both purée of tomato soup and purée of pea soup are made, to which are added carrots, white turnips and leeks to produce Mongole soup.
 
“Purée Mongole” is cited in English from at least 1889, in a recipe by a French chef. The origin and reasoning behind the name “Mongol(e)” is not known. Mongole soup has declined in popularity and is seldom seen today.
 
       
Wikipedia: Purée Mongole
Purée Mongole, also called Cream Mongole, is a creamed split pea-tomato soup of unknown origin that dates back to the at least the late 1800’s. Popular during the period between the 1920s–1940s, it is similar to boula.
 
Purée Mongole is usually made with carrots, onions, white turnips, leeks, a stock (either beef or chicken) and milk. Depending on the recipe, it can be seasoned with curry powder, salt, pepper, ground cloves, tumeric, nutmeg, cumin, and basil. Simplified recipes printed in many cookbooks of the time, including the 1946 edition of the Joy of Cooking, used canned, condensed pea and tomato soups as a base with additional vegetables and seasonings added.
 
Google Books
The Table:
How to Buy Food, How to Cook It, an How to Serve It
By Alessandro Filippini
New York, NY: Charles L. Webster & Company
1889
Pg. 158:
50. Purée Mongole.—Boil in a saucepan half a cupful of dried peas in two gills of white broth (No. 99), for one hour; if fresh peas, half an hour will be sufficient. Cut up in julienna shape, one medium-sized sound carrot, one small turnip, and one leek; place them in a saucepan with half an ounce of butter on the hoit stove, cover the pan, and let simmer for five minutes. Peel two good-sized ripe tomatoes, cut them into quarters, put them in a saucepan with a quarter of an ounce of butter; season with one pinch of salt and half a pinch of pepper, add one gill of white broth (No. 99). Let cook for twenty minutes on a brisk fire. Then strain the tomatoes through a fine sieve into a bowl, add them now to julienne, let al lcook five minutes longer; strain the peas through the sieve into the julienne, let the whole come to a boil, and serve in a hot soup-tureen.
 
Brooklyn Daily Eagle
1 September 1895, Brooklyn (NY) Daily Eagle, “First of Ocean Liners,” pg. 14:
...Puree mongole…
   
Google Books
(Note: The Google Books identical recipe is from European and American Cuisine by Gesine Lemcke, with copyrights 1895, 1897, 1903, 1914, 1923, 1924 and published 1933—ed.)
Standard American Cook Book
Springfield, OH: The Crowell & Kirkpatrick Co.
1897
Pg. 48:
MONGOLE SOUP.
Place a saucepan with half a pint of dried peas over the fire, cover with cold beef stock, boil until tender, and then pass through a sieve. Cut on medium-sized, well-cleaned carrot into thin slices, lay several slices over one another, and cut into strips like straws; cut one white onion the same way, and add to this one leek cut fine. Place the vegetables over the fire, with one tablespoonful of butter; cook ten minutes, then cover with one pint of boiling meat stock, and cook until tender. Stew one half of a can of tomatoes, with
1/2 tablespoonful of butter,
1/2 teaspoonful of salt,
1/4 teaspoonful of pepper,
1 teaspoonful of sugar.
For twenty minutes, and then strain.  As soon as the vegetables are done, add the pea puree, the strained tomatoes and one quart of good stock; cook for a few minutes, season to taste, and serve.
Mrs. Gesine Lemecke, Principal German-American Cooking College, Brooklyn, N. Y.
 
Chronicling America
31 December 1902, New York (NY) Tribune, pg. 7, col. 4 ad:
BROOKLYN’S PREMIERE RESTAURANT.
THE RONZO.
JOHNSTON BUILDING
8-10-12 Nevins St.
(...)
Potage Mongole
   
Google Books
Curling in Canada and the United States
By The Rev. John Kerr
Edinburgh: Geo. A. Morton
1904
Pg. 655:
SOUPS (...) Puree Mongole
     
14 November 1907, Boston (MA) Journal, pg. 9 ad:
Puree Mongole
(Dreyfus, 6 to 20 Beach Street—ed.)
         
Google Books
The Practical Hotel Steward
By John Tellman
Fourth Edition
Chicago, IL: The Hotel Monthly
1913 (Original copyright 1900)
Pg. 231:
Puree Mongol: Puree of split peas and tomatoes, vegetables cut in julienne, chervil. 
 
Google Books
The Golden Rule Cook Book
By Mrs. Maud Russel Lorraine Sharpe (Freshel)
Boston, MA: Little, Brown and Company
1919 (Copyright 1907, 1910)
Published in 1908 under title: Six hundred recipes for meatless dishes.
Pg. 59:
PUREE MONGOLE
Put 1 can of tomatoes in a saucepan and with it 2 cups of strong vegetable broth, 1 stalk of celery, 1 slice of onion, (Pg. 60—ed.) 1 bay leaf, 3 allspice, 3 cloves, salt and pepper, and let cook slowly for half an hour. Pour the liquid through a sieve, pressing with it as much of the tomato as will go, reserving the celery. Return to the saucepan, add 1 tablespoon of reduced vinegar, 1 tablespoon of boiled peas, 1 tablespoon of canned string beans, split in half, and the cooked stalk of celery shredded into thin strips two inches long; let simmer for five minutes, season with salt and pepper, add 1 tablespoon of butter, remove from the fire, and beat vigorously into the soup 1 well-beaten egg.
 
14 November 1925, Los Angeles (CA) Times, pg. A6:
Puree Mongole.
 
6 April 1926, Los Angeles (CA) Times, pg. A7:
Mongolian Soup.
     
8 February 1929, Fresno (CA) Bee, “Rector’s Recipes” by George Rector, pg. 8, cols. 1-3:
I have any number of delicious soup recipes, and I have in mind to give one for Mongole soup, which is made from a combination of puree of tomato and puree of pea with carrot, white turnip and leek cut en Julienne.
 
Puree Of Tomato Soup
Open a quart can of tomatoes and pour contents into a saucepan. Add two cupfuls of water, one scant tablespoonful of sugar, one small bay leaf, ten peppercorns, four whole cloves and one slice of onion. Bring to a boil, cover saucepan and simmer slowly for twenty minutes. Strain and season with one scant teaspoonful of salt. Melt three tablespoonfuls of butter and blend in three tablespoonfuls of flour until perfectly smooth. Then gradually add the strained tomato liquid to the butter and flour and bring to a boil. Serve with croutons.
 
Puree Of Pea Soup
Pick over one pint of dried peas and soak over night in cold water. Drain and put them in a saucepan with two and a half quarts of cold water, a half onion, a few celery leaves and a quarter pound of salt pork. A small ham bone may be used instead of the salt pork. Bring slowly to a boil and simmer until peas are soft enough to rub through a sieve—about three or four hours. Rub peas through a sieve and add the puree to three tablespoonfuls of butter that has been melted and blended with three tablespoonfuls of flour. Season with salt and pepper and dilute to proper consistencywith rich milk. Always add a few fresh green peas to a puree of pea soup.
 
Mongole Soup
Cut carrots, white turnips and leeks in Julienne slices and cook in boiling, salted water until tender, also cook a few green peas (fresh) in a separate saucepan. The peas wil ltake a little longer to cook, so start them in advance of the vegetables en Julienne. Combine equal portions of puree of tomato with puree of pea and as Mongole soup is to be served rather thick it may be necessary to thicken with roux (melted butter and flour). The method, of course, is to add the Mongole soup gradually to the roux and stir until perfectly smooth. Bring to a boil and add the cooked vegetables.
 
Tips on Tables:
Being a Guide to Dining and Wining in New York at 365 Restaurants Suitable to Every Mood and Every Purse
by George Ross
New York, NY: Covici Friede Publishers
1934
Pg. 37: MAISON ROYALE 6 East 52nd Street (5th and 6th Avenues)
But to get on to the gastronomical details, the Maison Royale’s French cuisine is something unique, and the chicken mascotte and lobster a la Newburg are its prides. But it’s sheer reportorial ineptitude to overlook the casaba melon; the potage mongole, heated over a little alcohol burner at your tableside; the sausage gastronome, an entree to delight any epicurean soul; and meringue Chantilly in the way of dessert.
 
28 September 1939, New York (NY) Times, pg. 21 ad:
Creamed Mongol Soup
(The Biltmore, Madison Avenue at 43rd Street—ed.)
 
13 December 1981, New York (NY) Times, “Soup Inspired by the Bible” by Craig Claiborne and Pierre Franey, pg. SM39:
Potage mongol
(Split-pea and tomato soup)
1 pound green or yellow split peas
1 pound lean pork bones, cut into two-inch pieces
1 cup coarsely chopped onion
1 clove garlic, finely minced
6 cups water
1 bay leaf
1/2 teaspoon dried thyme
4 cups imported canned tomatoes
Salt, if desired
Freshly ground pepper
2 carrots, trimmed and scraped
1 cup heavy cream
Tabasco sauce (...)
   
Google Books
Cooking in America, 1840-1945
By Alice L. McLean
Published by Greenwood Publishing Group
2006
Pg. 128:
PUREE MONGOLE
(Ellsworth, 1939)
Although virtually unheard of today, Mongole soup enjoyed wide celebrity in the 1930s. Most commonly a blend of canned split-pea and tomato soups, Mongole became so hip that it was served at the New York City restaurant “21,” from which the following version comes.
 
Cook a cup of carrots cut in shoestrings and a cup of fresh peas. Mix and heat a can of Campbell’s tomato soup and one of Campbell’s split pea; add the carrots, (Pg. 129—ed.) peas, and half a teraspoonful of onion juice and serve from a tureen, with bread sticks. Sherry, a scant tablespoonful to a serving, adds the final touch.