Preheat oven to 350 degrees. Cook macaroni until tender, drain. Melt butter in same pot, add onions and green pepper and saute over low heat 5 minutes. Add flour, salt and pepper and stir to blend well. Add milk and mushroom soup and stir over low heat until smooth. Add macaroni, flaked tuna, peas and 1/2 the cheese to sauce and stir to blend well. Pour into 2-quart buttered casserole, top with remaining cheese. Top cheese with 2 slices battered day-old bread cut in cubes, and sprinkle bread with paprika generously. Bake at 350 degrees for 35 minutes, or until nice and brown. This casserole reheats easily over low heat in saucepan, or in microwave at 50%. Like most casseroles, it tastes even better reheated.