Preheat the oven to 350°F. Cook the pasta according to the package directions; drain it and transfer it to a large bowl.
Stir in the sausage, soup, sauerkraut, 2 cups of the cheese, the milk, onions, mustard and garlic.
Spoon the mixture into two greased 8-inch square baking dishes, then sprinkle it with the remaining cup of cheese. Bake, uncovered, until golden brown and bubbly, 45 to 50 minutes.
