Cut the cauliflower and broccoli flowerets from the heads. Steam until just tender. Place the cooked vegetables in a large casserole dish. (Save the stems and leaves for stock.)

Meanwhile saute the onions and mushrooms in 1 Tablespoon of margarine until the onions are clear.

Meanwhile, melt the remaining 3 Tablespoons margarine in the microwave. Stir in the flour, salt, and pepper. Add the milk and microwave on "7" until thickened, stirring every 90 seconds. Add the cheese and stir until melted.

Add the cooked mushrooms and onions to the cheese sauce and pour over the steamed vegetables. Bake uncovered at 350 degrees for 20 minutes.