Saute onion in butter until tender. Heat green beans; drain (retain liquid). Add half of cheese to beans while hot and mix. Mix soup, mushrooms and liquid and onions in separate bowl. Add to beans and cheese, season with salt and pepper. Pour into large greased casserole and arrange onion rings on top. Bake at 350° for 35 to 40 minutes, until top is bubbling and onions are brown. Makes about 12 servings.
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