Microwave broccoli, cauliflour and sliced onion for 3 minutes in a casserole dish.
Make Cream Sauce by mixing flour and 1/4 cup of melted margarine together. Add milk until the sauce thickens. Remove and add cream cheese. Mix together. Spread over vegetables. Grate cheddar cheese over top.
Mix bread crumbs with 1 tbsp of margarine and spread over top.