Preheat oven to 450°.
Melt margarine in a saucepan over medium-high heat. Add onion; sauté 3 minutes. Add milk and soup. Cook 3 minutes; stir constantly with a whisk. Combine soup mixture, noodles, and next 6 ingredients (noodles through pimiento) in a 2-quart casserole. Combine breadcrumbs and cheese; sprinkle on top. Bake at 450° 15 minutes or until bubbly.