START noodles cooking according to directions on package.
In a large skillet over medium heat, MELT butter; ADD bell pepper, onion and mushrooms; SAUTE until tender, about five minutes.
In a small mixing bowl, WHISK together flour and milk until smooth; STIR into vegetable mixture in skillet; COOK, STIRRING constantly until thickened, about five minutes.
REMOVE from heat; ADD cheese; STIR until melted.
DRAIN cooked noodles; STIR together vegetable-cheese mixture, noodles, tuna, parsley, salt and pepper; SPOON into lightly greased 13 X 9-in baking dish.
BAKE, covered, at 350F for 25 minutes.
STIR together breadcrumbs and melted butter; SPRINKLE over casserole; BAKE five more minutes, uncovered.
Per Serving (excluding unknown items): 497 Calories; 30g Fat (54.9% calories from fat); 38g Protein; 18g Carbohydrate; 1g Dietary Fiber; 120mg Cholesterol; 991mg Sodium. Exchanges: 1/2 Grain(Starch); 4 1/2 Lean Meat; 1/2 Vegetable; 1/2 Non-Fat Milk; 4 1/2 Fat.