4. Step
Add cabbage and onion to the drippings in the pot; cook, stirring occasionally, until tender, about 5 minutes. Stir in sauerkraut, mustard, pepper and salt. Sprinkle with flour and cook, stirring constantly, until well incorporated, about 1 minute. Add milk and cook, stirring constantly and scraping to release any browned bits from the bottom of the pot, until simmering and thickened, about 4 minutes.