Saute muchrooms in the 1/2 cup butter until lightly browned.
Add the onion and cook until soft. Stir in the flour and cook until thick, stirring constantly. Add sour cream and whipping cream and stir well. Pour into a buttered casserole dish. Combine the melted butter and breadcrumbs and sprinkle over the casserole. Bake at 325* for 35 minutes or until lightly browned and bubbly.