Sauté mushrooms in sherry and 1 tablespoon 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. 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 13- x 9- x 2-inch baking dish coated with cooking spray.
Combine breadcrumbs and remaining Parmesan cheese and margarine; sprinkle over chicken. Bake, uncovered, at 350° for 30 minutes.