Tuna-Melt Casserole

3 cup(s) broccoli flowerets

2 tablespoon(s) margarine , substitute butter

2 tablespoon(s) all-purpose flour

3/4 teaspoon(s) salt

1/4 teaspoon(s) ground black pepper

4 cup(s) (2 percent) reduced-fat milk

1 cup(s) (4 ounces) shredded Swiss cheese

1 can(s) (12 ounces) chunk light tuna in water , drained and flaked

2 medium tomatoes , cut into 1/4-inch-thick slices