Polish Casserole

Ingredients

• 4 cups short pasta uncooked, such as penne or fusilli
• 2 tablespoons butter
• 2 tablespoons all-purpose flour
• 2 1/2 cups milk
• 1 cup sour cream
• 2 tablespoons Dijon mustard
• 1 teaspoon garlic powder
• 16 ounces sauerkraut rinsed and well-drained
• 1 1/2 pounds polish sausage smoked, or kielbasa, sliced 1/2-inch thick
• 3 cups Swiss cheese grated and divided
• 4 green onions chopped
• kosher salt
• freshly ground black pepper