Preheat oven to 400°. Cook noodles according to package directions. Drain noodles; set aside.
In a large bowl, combine soup, milk, mayonnaise and mustard. Stir in the mushrooms, tuna and red pepper. Add noodles to soup mixture and stir until blended. Transfer to a greased 8-in. square baking dish.
Combine topping ingredients; sprinkle over casserole. Bake until bubbly, 25-30 minutes.
