- Preheat oven to 350 degrees F.
- Cook noodles according to package directions, adding the green beans the last 3 minutes of cooking. Drain and set aside. Meanwhile, toss the bread crumbs with the 1 tablespoon melted butter; set aside.
- In a large skillet melt 2 tablespoons butter over medium heat. Add mushrooms, sweet pepper, onion, celery, and garlic. Cook and stir until vegetables are tender. Stir in flour, salt, mustard, and black pepper. Add LACTAID® Reduced Fat Milk all at once; cook and stir until thickened and bubbly. Stir in tuna, cooked noodles, and green beans.
- Spoon tuna mixture into a 1 1/2-quart au gratin dish or oval baking dish. Sprinkle bread crumb mixture atop casserole. Bake, uncovered, in a 350 degree F oven for 20 to 25 minutes or until heated through and bread crumbs are golden.