PREHEAT oven to 350° F. Lightly grease 13 x 9-inch baking dish.
COMBINE breadcrumbs and 2 tablespoons butter in small bowl. Stir in 1/4 cup cheese.
MELT remaining butter in medium saucepan over medium heat. Add onion; cook, stirring occasionally, for 1 to 2 minutes or until tender. Stir in flour, salt and pepper; cook, stirring constantly, for 1 minute. Remove from heat; gradually stir in broth. Return to heat; bring to boil over medium heat, stirring constantly. Stir in evaporated milk and remaining cheese; cook over low heat until cheese melts. Remove from heat. Stir in chicken, mushrooms, peas and sherry.
COMBINE pasta and chicken mixture in large bowl. Pour into prepared baking dish. Sprinkle with breadcrumb topping.
BAKE for 20 to 25 minutes or until topping is lightly browned. Serve immediately.