Cook penne to desired doneness as directed on package. Drain.
Meanwhile, heat oven to 350°F. Spray 2-quart casserole with nonstick cooking spray. Melt 2 tablespoons butter in medium saucepan. Stir in flour, salt, pepper and milk. Cook and stir over medium heat for 5 to 8 minutes or until mixture thickens. Remove from heat. Stir in cheeses until melted.
In sprayed casserole, combine cooked penne, onions, tomatoes and cheese sauce; mix well.
In small bowl, combine bread crumbs and 2 tablespoons butter; mix well. Sprinkle over penne mixture.
Bake at 350°F. for 20 minutes or until crumbs are golden brown.