Step 1

Cook and drain pasta according to package directions.

Step 2

Melt butter in medium saucepan over medium heat; add onion and garlic. Cook 4 minutes, stirring frequently. Add flour, salt and pepper; cook, stirring constantly, 1 minute. Stir in milk; bring to a boil. Reduce heat and simmer 1 minute, stirring frequently. Remove pan from heat; stir in 1 cup Cheddar cheese and the Parmesan cheese.

Step 3

Return pasta to cooking pot. Stir in sauce and sausage until pasta is well coated.

Step 4

Spread pasta mixture into 13x9-inch baking dish that has been sprayed with non-stick cooking spray. Arrange tomato slices over top. Top with remaining Cheddar cheese. Bake in preheated 350° F oven 30 minutes or until casserole is bubbling around edges and is hot throughout. Let stand 5 minutes before serving.