Heat oven to 350 degrees F. Coat a 2-quart oval baking dish with nonstick cooking spray.
Bring a large pot of lightly salted water to boiling. Add pasta and cook 10 minutes, according to package directions. Drain.
While pasta cooks, heat a large nonstick skillet over medium-high heat. Add sirloin, breaking apart with a wooden spoon. Cook 6 minutes, until no longer pink. Season with 1/4 teaspoon of the salt and the pepper. Remove from heat and stir in ketchup and mustard. Transfer to a bowl.
Return skillet to medium heat and add butter. Once melted, whisk in flour, then add milk in a thin stream. Bring to a simmer and cook 2 minutes. Remove from heat and whisk in remaining 1/4 teaspoon salt and 2 cups of the cheese. Stir in pasta. Pour into prepared dish. Top with meat mixture, remaining cheese, sliced tomatoes and bread crumbs. Spritz with nonstick cooking spray.
Bake at 350 degrees F for 20 minutes. Increase heat to broil and broil for 2 minutes.