Preheat oven to 350.
Melt butter in large skillet over med-high heat.
Add mushrooms and sauté for 5 minutes.
Add flour and cook for 1 minute.
Add milk, sour cream, and cheese.
Stir together; let it come to a bubble.
Add salt and pepper.
Add drained green beans and mix.
Transfer to 9x13 and spread onions on top.
Bake at 350 for 45 minutes.