Beef Stroganoff

1 pound round steak (cut into strips)
½ cup sliced onion
2 tablespoons butter
1 small can sliced mushrooms
2 cans cream of mushroom soup
½ cup sour cream
up to ⅓ cup water (optional)

Brown meat and onion in butter. Add mushrooms and soup. Cover and simmer 45 minutes. Add water depending on consistency, then add sour cream last. Heat through, but do not boil. Serve over egg noodles.