Preheat oven to 350°. Spray 6-cup casserole with non-stick cooking spray. Place potoatoes in a large saucepan; add water to cover. Bring to a boil over high heat. Boil 12 minutes or until potatoes are tender. Drain potatoes; discard liquid. Let potatoes stand 10 minutes or until cool enough to handle. Melt margarine in a small saucepan over medium heat. Add flour. Cook and stir 3 minutes or until small clumps form. Gradually whisk in milk. Cook 8 minutes or until sauce thickens, stirring constantly. Remove saucepan from heat. Stir in nutmeg, paprika and pepper. Cut potatoes into thin slices. Arrange half of the potato slices in prepared casserole. Sprinkle half of the onion slices over potatoes. Repeat layers. Spoon sauce over potato mixuture. Combine bread crumbs, finely chopped red onions and cheese into a small bowl. Sprinkle mixture evenly over sauce. Bake 20 minutes. Let stand 5 minutes before standing. Garnish as desired.