Cook beans according to package directions; drain.
Heat 1 tablespoon butter in large skillet; add breadcrumbs, stirring until toasted. Remove from skillet and set aside.
Melt 2 remaining tablespoons butter in skillet; stir in flour, salt and pepper. Heat until mixture bubbles; gradually add mustard and sour cream and blend well. Heat thoroughly but do not boil. Add beans, cheese, and onion; toss gently until well mixed.
Spoon into a greased 1 1/2 quart casserole. Top with reserved bread crumbs. Bake at 350F for 10 minutes.