Preheat oven to 350 degrees. Cook macaroni al dente and set aside. Meanwhile, melt butter in a large skillet over medium heat, add onions and peppers and cook until soft. This should take about 2-3 minutes. Whisk in flour, salt and pepper. Stir until smooth. Add milk and soup and cook over low heat until the sauce begins to thicken. Stir in the cooked macaroni, tuna, peas, and 1/2 of the cheese. Stir until everything is thoroughly coated. Turn off the heat. Pour the mixture into a 2 qt. buttered casserole. Top with the remaining cheese and bread crumbs. Bake until the casserole is hot and bubbly and the top is golden brown, about 35 minutes.