Prepare green beans according to package directions; drain.
Meanwhile, in large skillet, melt butter over medium-high heat. Add onions and mushrooms; salt and pepper to taste. Sauté until onions are soft and all liquid has evaporated from mushrooms.
Reduce heat to medium-low; add soup, cheese and sour cream, stirring until cheese has melted. Mix in green beans and half the fried onions.
Transfer to a greased 13x9-inch baking dish. Bake, uncovered, in a preheated 350°F oven 30 minutes or until bubbly, stirring halfway through. Sprinkle remaining fried onions on top; continue baking 5 minutes or until golden brown.