Heat oven to 200C/fan 180C/gas 6. Melt half the butter in a non-stick pan. Cook the onion for 3-4 mins, then increase the heat and add the mince. Fry for a further 3-4 mins until the beef has browned. Stir in the stock, Worcestershire sauce, tomato puree and some seasoning. Gently bubble for 15-20 mins until the mince is tender and the sauce has thickened.
To assemble, cut the jacket potato in half lengthways and scoop the flesh into a small bowl, leaving the skin intact. Mash the potato with the remaining butter and season well. Divide the mince between the potato skins, then cover with the mash. Transfer the potatoes to a baking dish, sprinkle with cheese, then bake for 15-20 mins until golden. Serve with your favourite veg.