Cook pasta according to package directions, omitting salt and fat. Set aside.
Melt 1 tablespoon light butter in a large skillet over medium-high heat. Add chicken, and cook 2 minutes on each side or until lightly browned. Stir in sherry and 1/2 cup water. Cover, reduce heat to low, and simmer 20 minutes or until done. Drain and reserve liquid. Cut chicken into 1-inch pieces; set aside.
Melt remaining 1 tablespoon light butter in skillet. Saute onion and mushrooms over medium-high heat 5 minutes or until tender.
Stir together flour and l/2 cup broth. Add reserved liquid, flour mixture, and remaining 1 1/2 cups broth to skillet. Cook, stirring often, over medium-high heat until thickened. Remove from heat. Stir in chicken, sour cream, Monterey Jack cheese, 1/4 cup Parmesan, and pepper.
Spoon mixture into a lightly greased 13- x 9-inch baking dish. Sprinkle with remaining 1/4 cup Parmesan cheese and breadcrumbs.
Bake at 350° for 30 minutes or until thoroughly heated.