1. Preheat oven to 375° F.
2. Place half of the cooked macaroni in a 1 1/2-quart casserole sprayed with vegetable pan spray. Sprinkle with half of onion and cheese. Repeat layers.
3. Melt margarine in small saucepan. Remove from heat, stir in flour. Add milk slowly, stirring until smooth. Add salt and pepper. Return pan to heat; cook, stirring constantly, until sauce thickens. (This will be a thin sauce.)
4. Pour over macaroni. Cover. Bake for 30 minutes. Uncover. Bake 15 minutes longer.