Prepare chicken broth in advance, then cool completely and refrigerate. Measure out 800 ml of the broth. Cut the chicken into cubes and refrigerate until ready to use.
Skim the fat from the cooled chicken broth and set aside. Heat the margarine over medium-high heat for about 2 minutes until it stops foaming. Remove from heat and stir in the chicken fat until dissolved. Stir in the wheat flour. Return to medium heat, pour in the 800 ml chicken broth while stirring constantly, and simmer for 10 minutes, stirring occasionally.
Drain the asparagus and mushrooms in a strainer, reserving the asparagus liquid. Cut the asparagus into roughly 2 cm pieces.
Stir the curry powder into the sauce, then add half of the reserved asparagus liquid and the sour cream. Bring to a boil over medium-high heat. If the sauce is too thin, mix 1 tablespoon cornstarch with a little of the reserved asparagus liquid, stir it in, and bring back to a boil. Season with salt and adjust the curry powder or sour cream to taste for heat level and creaminess.
Gently fold in the asparagus, mushrooms, and chicken pieces. Heat through over low heat for 2–3 minutes without boiling.
Serve with rice, or optionally stir the rice into the pot.