Cook spaghetti 1 minute less than al dente.
Cook sausage in butter until browned.
Stir in onion, garlic, seasoning, and tomatoes.
Sprinkle flour, whisk in milk, and simmer until thickened.
Stir until wilted.
Toss pasta with sauce, transfer to baking dish, top with mozzarella, and bake 18–20 minutes at 375°F.
