Goulash with Sausage and Sauerkraut

Name: Goulash with Sausage and Sauerkraut
Ingredients: 3/4 pound Hungarian sausage, such as kolbász (smoked with paprika), sliced crosswise 1/2 inch thick; 1 1/2 pounds boneless beef chuck, cut into 1 1/2-inch cubes; 1 1/2 pounds boneless pork shoulder, cut into 1 1/2-inch cubes; 1/3 cup bacon fat or vegetable oil; 4 large onions (4 pounds), finely chopped; 1/3 cup sweet Hungarian paprika; 1 pound sauerkraut, drained; 3 cups chicken stock or canned low-sodium broth, plus more, if needed; 4 bay leaves; 1/2 teaspoon dried thyme; Salt and freshly ground pepper; 1 cup sour cream, plus more for serving; Snipped chives, for sprinkling