Preheat the slow cooker if necessary; see the manufacturer’s handbook.
Heat the oil in a frying pan, add the beef a few pieces at a time until all the meat is in the pan, then fry over a high heat, stirring, until browned. Remove the beef from the pan with a slotted spoon and transfer to the slow cooker pot.
Add the onion to the pan and fry, stirring, for 5 mins or until softened and just beginning to turn golden. Stir in the flour, then gradually mix in the stock. Add the Worcestershire sauce, tomato purée, mustard and leaves from 2 sprigs of the rosemary. Season with salt and pepper and bring to the boil, stirring.
Add the diced vegetables to the slow cooker pot. Pour the onion and sauce over them, then cover with the potato slices, arranging them so that they overlap and pressing them down into the stock. Sprinkle with the leaves torn from the remaining stem of rosemary and a little salt and pepper.
Cover with the lid and cook on high for 7–8 hrs until the potatoes are tender. Lift the pot out of the housing using oven gloves; dot the potatoes with the butter and brown under a preheated hot grill, if liked.