Put the root vegetables, potato, bay leaves and thyme in the slow cooker. Put the beef in a bowl and season with salt and pepper. Add the flour and mustard powder. Toss gently, making sure the beef is coated in the flour mixture. Shake off and reserve any excess mixture.
Heat the oil in a large frying pan over a high heat. Reduce the heat to medium, add the beef and fry for 5–8 minutes