Heat oven to 350°F. Grease 8-inch square (2-quart) baking dish. Place uncooked linguine in colander; rinse with hot water for 15 to 30 seconds or until softened. Set aside.
Melt 1 tablespoon margarine in medium saucepan over medium heat. Add mushrooms and onions; cook and stir 2 to 3 minutes or until tender.
Stir in flour, salt and pepper; mix well. Stir in broth; cook and stir until thickened and bubbly. Stir in half-and-half, sherry, chicken, roasted bell peppers and linguine; mix well. Spoon mixture into greased baking dish; cover with foil. Bake at 350°F. for 15 minutes.
Meanwhile, in small bowl, combine all topping ingredients; mix well. Uncover dish; sprinkle topping over casserole. Bake, uncovered, an additional 5 to 10 minutes or until casserole is bubbly and topping is light golden brown.