Preheat oven to 400 degrees Fahrenheit. In a large saucepot, cook pasta in boiling salted water for 5 minutes; add broccoli to pasta and cook another 5 minutes or until broccoli is tender and pasta is al dente. Drain well and return to saucepot; set aside.
Meanwhile, in a 3-quart saucepan, melt butter or margarine over low heat. Stir in flour, salt, and pepper until blended and cook, stirring constantly, 1 minute. Gradually stir in milk; increase heat to medium-high and cook, stirring occasionally, until mixture thickens and boils. Boil 1 minute, stirring frequently. Remove saucepan from heat and stir in 1 cup of cheese until blended.
Add cheese sauce and tuna to pasta and broccoli in saucepot; toss until evenly mixed. Transfer mixture to a 13"x9" glass baking dish. Arrange tomato slices on top, overlapping if necessary. Sprinkle with remaining 1/2 cup cheese.
Cover baking dish with foil and bake 20 to 25 minutes or until hot and bubbly. Makes 6 main-dish servings.