Preheat oven to 350 degrees F.
Grease a 2-qt. ovenproof casserole.
Combine 2 T. melted butter and flour in a saucepan.
Cook over low heat until bubbling.
Blend in salt, pepper, onion, and sour cream.
Mix in cooked green beans.
Place in prepared casserole.
Combine cheese, bread crumbs, and remaining (2 T.) melted butter.
Sprinkle over bean mixture.
Bake in preheated oven for 20 to 25 minutes.
Yield: 10 servings.