Preheat oven to 350 degrees F. Cook noodles according to package directions; drain and set aside.
Meanwhile, if desired, in a small bowl toss panko with butter; set aside.
In a large saucepan combine green beans, mushrooms, sweet pepper, onion, celery, the water, and garlic. Bring to boiling; reduce heat. Simmer, covered, about 5 minutes or until vegetables are tender.
Stir cream of mushroom soup and milk into vegetable mixture. Cook and stir until heated through. Remove from heat. Add cheese, stirring until melted. Stir in cooked noodles and tuna.
Spoon mixture into an ungreased 2-quart casserole. If desired, sprinkle panko mixture around outside edge of casserole. Bake for 25 to 30 minutes or until tuna mixture is bubbly and panko mixture is golden brown. If desired, top with fish-shape crackers.