Cook noodles in salted water; drain and set aside.
Sauté onions and garlic in butter until onions are tender. Add flour; blend until smooth. Add milk and cook, stirring constantly, until thickened and smooth. Add tuna, vegetables, and salt and pepper to taste; stir in cooked noodles.
Pour tuna noodle mixture into a 1 1/2-quart casserole. Mix crumbs and dried parsley with melted butter; sprinkle over casserole. Bake at 350° for 20 minutes, or until heated through. Tuna Noodle Casserole serves 6.