Lay the beef slices flat and season with salt and pepper. Place a spoonful of sausage meat or stuffing on each slice and roll tightly. Secure with toothpicks or string.
Heat a little oil in a pan and sear the beef olives until browned on all sides - this step adds depth to the flavour.
Mix the beef stock, tomato purée, Worcestershire sauce, and mustard in a bowl.
Place the beef olives in the slow cooker, pour over the sauce, and cook on low for 6-8 hours or high for 4-5 hours.
Stir in the flour or cornstarch if you prefer a thicker gravy.