Preheat oven to 350. Cook cauliflower for 4 minutes, drain. Place in a 2 quart greased casserole dish. Saut? onions in 1 tblsp of butter until tender. Blend in flour, salt and pepper. Stir in sour cream, cook until thick, and pour over cauliflower. The mixture can be refrigerated overnight at this point and finished before cooking. Melt remaining butter, stir in bread crumbs. Sprinkle over broccoli and top with cheese Bake for 25-30 minutes.