Preheat oven 350°F. Lightly butter a 3-1/2 quart baking dish.
Remove casings from sausage. In a skillet, over medium heat, add sausage, crumble and cook until no longer pink and lightly browned. Add onion and sauté until softened.
In a large saucepan, melt butter over medium heat. Stir in flour, salt, pepper and dry mustard until smooth.
Gradually whisk in milk. Cook and stir until thickened. Add cheeses and stir until melted. Mix in sausage and onions.
Cook pasta according to package directions; drain. Stir into sausage mixture. Bake, uncovered, for 30-35 minutes or until top is golden brown.
Recipe Notes
Tomato wedges can be pressed partially into top of mixture before baking.
1/4 cup each of fresh basil leaves and fresh flat-leaf parsley, chopped, can be added to the cream sauce for additional Italian flavor.