Melt 1/4 cup butter in a large skillet over medium heat; add bell pepper, onion, and mushrooms, and saute 5 minutes or until tender.
Whisk together flour and milk until smooth; stir into vegetable mixture, and cook, stirring constantly, 5 minutes or until thickened. Remove from heat; add cheese, stirring until melted.
Stir in tuna and next 4 ingredients; spoon into a lightly greased 13- x 9-inch baking dish.
Bake, covered, at 350° for 25 minutes. Stir together breadcrumbs and 2 tablespoons melted butter; sprinkle over casserole, and bake 5 more minutes.