Preheat oven to 350 degrees F. Cook pasta as package directs; drain well. Meanwhile, for cheese sauce, in large saucepan, cook onion in 2 tbsps butter until tender, about 5 minutes. Stir in flour and pepper. Add milk all at once. Cook and stir constantly until mixture is slightly thickened and bubbly. Add cheeses and stir until melted. Stir in cooked pasta, 1 cup crumbled bacon and diced tomatoes. Spoon mixture into a lightly greased 2 quart baking dish. Sprinkle with 1/4 cup seasoned bread crumbs and remaining bacon. Melt remaining butter and drizzle over the bread crumbs and bacon. Bake at 350 degree F for 30-35 minutes or until hot and bubbly. Serves 8