Method:
Preheat oven to 375°F. Spray a baking dish with cooking spray; set aside. Cook noodles according to package directions. Drain well.
Meanwhile, melt butter in a medium saucepan. Stir in flour, and cook, stirring constantly, for 1 minute, or until the mixture bubbles but does not brown. Slowly pour milk into flour mixture, stirring constantly, until thick and bubbly, about 3 to 5 more minutes. Remove from heat, cover and set white sauce aside.
Heat 2 tablespoons of the oil in a large skillet over medium high heat. Add peppers, mushrooms, onions, salt and pepper and cook for about 7 minutes, or until mushrooms give up their liquid. Add tuna and cooked noodles and toss to combine. Transfer to baking dish and pour reserved white sauce over the top, covering it completely. Season with salt and pepper.
In a medium bowl, mix together bread crumbs and remaining 1 tablespoon oil. Arrange bread crumbs evenly over top of white sauce and bake for 20 to 25 minutes, or until hot and bubbly.