Sauté mushrooms in sherry and 1 Tbl margarine in a nonstick skillet over high heat 5 minutes.
Place chicken in a saucepan; add water to cover. Bring to a boil; cover, reduce heat, and simmer 20 minutes (or use equivalent amount of leftover chicken [or turkey]). Drain; let cool, and shred. Coat a saucepan with cooking spray; place over medium heat until hot. Add onion and sauté until tender.
Combine flour and 1/2 cup broth. Add 2 1/2 cups broth and flour mixture to onion. Cook over medium heat until thickened. Remove from heat; stir in sour cream.
Cook linguine according to package directions, omitting salt and fat; drain.
Combine mushroom mixture, chicken, sour cream mixture, Monterey Jack cheese, 1/4 cup Parmesan cheese, and pepper. Stir in linguine.
Spoon into a 13x9x2-inch baking dish coated with cooking spray.
Combine breadcrumbs and remaining Parmesan cheese and margarine; sprinkle over chicken.
Bake, uncovered at 350°F for 30 minutes.