Everyone at my dinner parties always raves about this dish.

1 pound ground beef
1 small onion, chopped
1 can (10.5 ounces) cream of mushroom soup
1 cup sour cream
1 can (4 ounces) sliced mushrooms, drained
1/2 teaspoon salt
1/4 teaspoon black pepper
8 ounces wide egg noodles, cooked and drained
1 cup shredded cheddar cheese
1/2 cup bread crumbs
2 tablespoons butter, melted