Fixens (or “fixins” or “fixings”) are trimmings, the sides of any dish, or food itself. “Chicken fixings” are prepared chicken. The terms became popular about 1837.
 
 
(Oxford English Dictionary)
fixing, vbl. n.
In pl. (orig. U.S.) Apparatus, equipment; trimming of a dress; the adjuncts to any dish, garnishing. Also (Austral. slang), strong liquor (Barrère and Leland 1889). Also transf.
1820-1 R. FLOWER Let. in R. G. Thwaites Early Western Trav. (1904) X. 126 ‘There wife,’ said he, ‘did you ever see such fixings?’ He felt the paper, looked in a mirror.., and gazed with amazement. 1827 J. F. COOPER Prairie I. ii. 30 ‘Your fixen seem none of the best for such a calling.’ 1839 MARRYAT Diary Amer. Ser. I. II. 228 White wheat and chicken fixings.
   
(Oxford English Dictionary)
chicken-fixings local U.S., chicken prepared as food; also fig.
1838 E. FLAGG Far West II. 72 Wheat-bread and *chicken fixens, or cornbread and common doins? 1854 M. J. HOLMES Tempest & Sunshine v. 70 We don’t have any of your Chicken Fixins nor little three-cornered handkerchiefs laid out at each plate.
   
(Dictionary of American Regional English)
fixing n Pronc-spp fixin’, fixen
(...)
2 pl; Food.
1828 Western Souvenir 147, I feel powerful weak; but I don’t like the fixens here, no-how.
(...)
3 pl; Trimmings, extras, frills—used esp. of foods.
1840 Hoffman Greyslaer 2.58 NY, Add the salt and pepper fixings, and the king himself hasn’t a slicker supper.
   
(Dictionary of American Regional English)
chicken fixings n pl [fixing]
Chicken prepared as food; elaborate or “fancy” food. formerly W Midl old-fash CF common doings, doing
1838 Flagg Far West 2.72 IL, The first inquiry made of the guest by the village landlord is the following: “Well, stran-ger, what’ll ye take: wheatbread and chicken fixens, or corn-bread and common doins? 1843 (1916) Hall New Purchase 303 IN, A snug breakfast of chicken fixins, eggs, ham-doins and corn slap-jacks.
 
December 1822, The Port-Folio, pg. 8:
“I feel powerful weak—but I don’t like the fixens here, now how—I’m a ‘bominable bad hand among women—so I’d thank ‘em not to be cutting their shines about me.”
   
Newspapers.com
21 August 1837, The Evening Post (New York, NY), pg. 2, col. 2:
BREAKFAST IN ALABAMA.—The following conversation is said to have taken place somewhere in that State:
 
Marm what do you charge for breakfast here? Why, if you have corn bread and common trimmins, it will be two bits (25 cents). But if you have wheat bread and chicken fixin’s it will be three bits. Let’s have the chicken fixins.
 
Newspapers.com
4 November 1837, The Columbia Democrat (Bloomsburg, PA), pg. 4, col. 5:
Breakfast in Alabama.—The following conversation is said to have taken place somewhere in that State:
 
“Marm what do you charge for breakfast here!” “Why, if you have corn bread and common trimmens, it will be two bits, (twenty-five cents.) But if you have wheat bread and chicken fixens. it will be three bits.” “Let’s have the chicken fixens.”
 
25 August 1838, Spirit of the Times, pg. 221:
CAT FISHING IN THE OHIO
Is described as “none of your corn-cakes and common doings,” and by many of our brother anglers in the West is considered “white bread and chicken fixins” when compared with Trout fishing at the North.
   
Jan/Feb 1839, American Turf Register and Sporting Magazine, “Trout Fishing on Long Island,” pg. 12:
Still, get in at Liff’s if possible; the way you will live there is “none of your common doings,” but if you cannot, why go to Sam’s, and that, let us tell you, is a huckleberry above “your corn bread and chicken fixins.”
   
Newspapers.com
22 July 1839, The Morning Post (London, UK), pg. 6, col. 6:
“In the West, when you stop at an inn, they say:
 
“‘What will you have? Brown meal and common doings, or white wheat and chicken fixings’—that is, ‘Will you have pork and brown bread, or white bread and fried chicken?’”