1. Begin by preheating the oven to 200°C (180°C fan/gas mark 6).
2. Prick the potatoes all over with a fork, before rubbing them with half of the olive oil. Place on a baking tray and season with plenty of salt and pepper.
3. Bake the potatoes in the oven for about one hour and 15 minutes, until crisp on the outside and soft in the middle.
4. Place the remaining oil and 1tbsp of butter in a large frying pan over a low heat and add the chopped leek.
5. Cook the leek for around 20 minutes, until very soft and golden brown, and then set aside to cool.
6. Once your potatoes are cooked, cut them in half lengthways. You may wish to let your adult do this bit as they will be very hot.
7. Scoop out the potato into a bowl, leaving the skins with a very thin layer of potato remaining.
8. Mash the potato with the remaining butter (and the double cream, if using), seasoning well with salt and pepper.
9. Stir through the leek, most of the cheese and the mustard (if using).
10. Spoon the mixture back into the potato skins and sprinkle the rest of the cheese over the top.
11. Return to the oven for around 15 minutes, until the cheese has melted and is beginning to turn golden.
Our loaded potato skins make a wonderful accompaniment to a barbecue outdoors, or why not have them alongside a simple, yet delicious salad?