Heat oven to 400. Spray a 9" x 13" casserole dish w/ cooking spray. Cook pasta as directed, drain, return to pot.
In saucepan, over low heat, melt butter. Stir in flour, salt and pepper. Cook 1 min, stirring constantly. Slowly whisk in milk. Increase heat to med-high. Cook until thickened, stirring.
Remove pan from heat. stir in 1-1/2 cup cheese until melted. Add cheese sauce to macaroni. Add tuna to macaroni mix. Stir well. Transfer to casserole dish. Place tomato slices on top of casserole. Sprinkle w/ remaining cheese. Cover w/ foil. Bake 20 mins. Remove foil, bake additional 10 mins.