Put the potatoes in the microwave on high (750W) for 20 minutes until cooked through. While theyre cooking, tip the mince and onion into a pan and cook on the hob for 10 minutes until the onion is golden and the mince has lost its pink colour. Splash in the Worcestershire sauce and cook for 1 minute, then pour in the stock and squeeze in the tomato pure.Give a good stir to dissolve the pure, then cover and simmer for 20 minutes or until the meat is tender.
Cut each potato in half and scoop the flesh into a bowl, leaving a thin layer inside the potato skin. Break up the flesh with a fork, then season and mash in the milk and butter until smooth and creamy.
Preheat the grill to high.
Put the potatoes skin side down in a shallow roasting tin and spoon in the meat mixture. Top with the mash and sprinkle with the cheese. Grill until the cheese has melted and the filling is hot.
