Place flour in a bowl; add broth, milk, and pepper. Stir the flour mixture well with a whisk.
Melt margarine in a nonstick skillet over medium-high heat. Add mushrooms and bell pepper; sauté 3 minutes. Add flour mixture. Cook 2 minutes or until thick and bubbly; stir constantly. Combine sauce, noodles, ham, cheese, and sherry in a 1 1/2-quart casserole or 10-inch round gratin dish; sprinkle with breadcrumbs. Bake at 450° for 10 minutes or until bubbly.