1. Preheat oven to 375°.
2. Heat oil in a medium Dutch oven over medium heat. Add onion; cook 5 minutes or until tender, stirring often.
3. While onion cooks, cook pasta according to package directions, omitting salt and fat. Drain.
4. Combine cornstarch and 1/2 cup milk in a small bowl. Add remaining 3 1/2 cups milk to onion; bring to a boil. Gradually stir in cornstarch mixture. Cook 2 minutes or until sauce thickens, stirring constantly. Remove from heat, and stir in salt, pepper, and nutmeg. Add cheddar and mozzarella cheeses, stirring with a whisk until cheeses melt. Stir in pasta. Pour pasta mixture evenly into 6 (2-cup) gratin dishes or ramekins coated with cooking spray.
5. Combine Parmesan cheese and breadcrumbs; sprinkle over pasta mixture. Bake at 375° for 30 minutes or until sauce is bubbly and top is brown.