Heat the oil in a frying pan, add the beef then fry over a medium heat until brown. Scoop out with a slotted spoon and transfer to the slow cooker.
Add the onion to the pan and fry, stirring for 5 mins or until softened. Stir in the flour, then gradually add the stock. Once all combined, add the Worcestershire sauce, tomato puree, mustard and rosemary. Season and bring to a boil, turn down and simmer
While the sauce is simmering, add the vegetables to the slow cooker. Pour the onions and sauce into the slow cooker and stir.
Cover with the potato slices, arranging them so they overlap. Sprinkle with some rosemary and season with salt and pepper.
Cover and cook on a high for 6 hours.