Preheat oven to 375F. Cook noodles according to package directions. Drain; return noodles to pot. For sauce: In a medium saucepan melt butter and cook celery and onion until tender. Stir in flour, mustard and pepper. Add milk all at once; cook and stir until slightly thickened and bubbly, whisking to remove any lumps. Gently fold sauce, tuna and peppers into noodles. Transfer to lightly greased 1.5-quart baking dish. Bake, uncovered, for 30-35 minutes or until heated through. Let stand 5 minutes.