1 can (4 ounces) sliced mushrooms
1 cup evaporated milk, undiluted
1/4 cup butter or margarine
2 tablespoons all-purpose flour
1/2 teaspoon salt
1/8 teaspoon pepper
1 can (7 ounces) tuna, drained and flaked
1 1/2 cups thin spaghetti (about 4 ounces), cooked
1/4 cup grated Parmesan cheese
1 cup soft bread cubes
Drain mushroom liquid into a measuring cup; add water to make 1 cup liquid. Add to the milk. Melt 2 tablespoons of butter or margarine in a large saucepan; blend in flour, salt and pepper. Stir over low heat until smooth; gradually stir in milk mixture and continue stirring and cooking over low heat until thickened. Add tuna, mushrooms and spaghetti; transfer to a 1 1/2-quart casserole; sprinkle with Parmesan cheese. Melt remaining 2 tablespoons of butter or margarine; toss with bread cubes. Arrange bread cubes over top of casserole.
Bake casserole, uncovered, at 350° for about 25 to 30 minutes.