Preheat oven to 300 degrees.
Boil water in large pot, add tortellini, cook for 5 minutes. Drain and place tortellini’s in bottom of a lightly buttered 9x13 baking dish. Sprinkle half of the Monterey Jack cheese over tortellini’s.
Toss chicken pieces in flour. Melt butter in large fry pan add floured chicken to sizzling butter. Lightly brown chicken in butter. Do chicken in shifts if needed. Removing chicken to a plate then adding more. Then transfer chicken pieces to baking dish. Evenly distributing them over noodles.
Add chopped onions to the same pan, cook until transparent. Add bouillon, chicken broth, sugar, cook for 2 minutes then add mushrooms to pan, and cook for 2 minutes more.
Add remaining cheese to to sauce, lightly stir with spatula until cheese is melted. Remove pan from heat and add sour cream and blend until smooth and creamy. Pour mushroom sauce over chicken and tortellini.
Cover dish with foil, bake for 45 minutes.