Cook tortellini 5 minutes less than time on package; drain. Place in lightly greased 8 1/2 x 11 inch dish. Pour over pasta 4 ounces of cheese. Lightly brown chicken, cut up, coated in flour, in the butter. Remove chicken and put over prepared tortellini. Saute onion in remaining butter. Add chicken broth, bouillon, sugar, and mushrooms. Boil about 5 minutes. Add cheese and stir until melted. Add sour cream. Blend. Pour over pasta and chicken. Cover with foil. Bake 45 minutes in 300 degree oven.