Entry in progress—B.P.

Midwest Living
Midwest Soups State by State
By Diana McMillen. Photographs by Mark Thomas
Missouri: Kansas City Steak Soup
The Show Me State stockyards once helped deliver some of the finest steaks to the world. Our version of the KC steak soup uses lean ground sirloin mixed with veggies and seasoned with a blast of steak sauce and Worcestershire. It cooks in just 25 minutes.

RecipeSource
Plaza III Steak Soup
Recipe By : From the Plaza III Restaurant in Kansas City
Serving Size : 10 Preparation Time :0:00
Categories : Beef Soups & Stews
Amount Measure Ingredient—Preparation Method
-------- ------------ --------------------------------
1/2 cup butter or margarine
1/2 cup all-purpose flour
8 cups hot water
2 pounds lean ground beef
2 tablespoons beef bouillon granules
1 cup onions—chopped
1 cup carrots—diced
1 cup celery—sliced
2 cups frozen mixed vegetables
28 ounces canned tomatoes—diced
1 teaspoon pepper—or to taste
1 tablespoon Accent® seasoning mix—(optional)
(...)
NOTES : I found this recipe in the Jubilation Cookbook by The Junior League of Toronto (1985)

Google Books
Specialty of the House:
A collection of recipes from the finest restaurants around the world compiled with the cooperation of the American Express Card
By Sandy Lesberg
Englewood Cliffs, NJ: Prentice-Hall
1970
Pg. 15:
Plaza III
Kansas City, Missouri
PLAZA III STEAK SOUP

18 July 1974, Los Angeles (CA) Times, Culinary by Rose Dosti, part VI, pg. J8:
Steak Soup originated with the Gilbert/Robinson Hotel chain in Kansas City and. pops up cross-country, wherever its…