Preheat oven to 375. Lightly grease a 3 quart baking dish. Cook noodles as pkg. directs. Drain; return noodles to pan.
For sauce, in large pan, melt butter over medium heat. Add onion and celery, cook an stir til vegetable are tender. Stir in flour, mustard, pepper and salt. Add milk all at once. Cook and stir til sauce is thickened and bubbly. Whisk to remove any lumps.
Gently fold sauce, tuna and tomatoes into noodles. Transfer noodle mixture to prepared dish. If desired top with chips.
Bake about 20 minutes or til heated through. Let stand for 5 minutes before serving. Makes 8 servings.