1. Preheat oven to 400F. Grease a 2-quart shallow baking dish.
2. Cook pasta according to package directions and drain.
3. Meanwhile, heat butter in large saucepan and add the onion; cook 1 minute until onion is soft. Stir in flour until combined. Whisk in hot milk and cook over medium heat until thickened; stir in mustard.
4. Add cheeses to sauce and cook, stirring constantly, until melted and smooth. Remove from heat. Add the pasta and stir to coat. Pour mixture into prepared baking dish.
5. Combine bread crumbs, Parmesan cheese and melted butter; sprinkle over pasta mixture in baking dish.
6. Bake until bubbly and topping is browned, about 30 min.