Melt butter in large skillet. Saute onions and mushrooms in butter. Boil green beans in chicken broth for 10 minutes and drain. Add green beans, mushroom soup, onion rings, and seasoning salt to onion mixture. Stir well. Pour into a greased 1 1/2 quart baking dish. Bake for 20 minutes, then top with cheddar cheese and bake for 10 minutes longer, or until casserole is hot and cheese is melted and bubbly.