Set a piece of rouladen flat on a work surface. Brush the top of it with 1 tsp. of the mustard. Set a carrot and celery stick at one end of the meat and top with 2 Tbsp. of sauerkraut. Roll into a cylinder and secure with string or a toothpick. Repeat with the remaining rouladen.
Heat the oil in a large skillet set over medium-high heat. Add the rouladen, in batches if necessary, and brown on all sides. Set the rouladen in your slow cooker. Drain the excess oil from the skillet before adding the stock/flour mixture and rosemary. Bring this to a boil and cook until thickened, about 2 minutes. Pour this mixture over the rouladen. Cover and cook on the low setting for 8 hours.
Note: This recipe is designed for an oval-shaped slow cooker with a 5 litre or more capacity. Thinly sliced beef for rouladen is available in the meat showcase.