Cook noodles according to package directions; drain and set aside.
Meanwhile, drain and flake tuna. Toss the bread crumbs with margarine or butter; set aside.
In a large saucepan combine frozen green beans, mushrooms, green or red pepper, onion, celery, water, bouillon granules, and garlic. Bring to boiling; reduce heat. Cover; simmer about 5 minutes or until vegetables are tender.
Meanwhile, stir together milk, cornstarch, and dry mustard. Stir into vegetable mixture. Cook and stir until slightly thickened and bubbly. Remove from heat, then stir in cheese until melted. Stir in noodles and tuna.
Spoon mixture into a 2-quart casserole. Sprinkle bread crumb mixture around outside edge of casserole. Bake, uncovered, in a 350 degree F oven for 25 to 30 minutes or until bread crumbs are golden. Makes 6 servings.