If you're vegetarian don't be put off by the Worcestershire Sauce in this recipe. The original Worcestershire Sauce does contain anchovies, but there are numerous brands these days offering up a vegan alternative (Biona, Annie's Naturals...). Or you could make your own - try this enticing recipe from the Viet Vegan.
Prick the potato skins in several places and bake for approx 1 hour 45 minutes at 180C/ 350F/ GM4 (until soft all the way through when poked with a sharp knife)
Remove from the oven and turn the heat up to 200C/ 400F/ GM6
Melt the butter in a large saucepan, slice the leeks thinly, add to the pan and cook over a low-medium heat for 5-10 minutes until soft. Stir frequently to prevent browning. Remove from the heat
Cut the potatoes in half, scoop out the flesh and mash until smooth - I used a
Add to the leeks along with the mustard, Worcestershire Sauce, and egg yolks
Also add the cheddar - reserving a handful to sprinkle over the potatoes prior to baking
Mix all ingredients together until fully incorporated. Taste and add salt & pepper as desired
Divide the mashed potato mix between the potato skins, scatter the remaining cheese on top and bake for 20 minutes until golden
Alternatively, let cool completely and store in the fridge until required then bake for 25-30 minutes.
See notes for freezing
Notes
These potatoes can be frozen. Let cool completely, then wrap each half individually in foil (minus the scattering of cheese). Label and freeze until required. Preheat your oven to 220C/ 425F/ GM7. Unwrap the potatoes , place on a baking sheet, cover with foil and bake for 40 minutes. Remove the foil, scatter with cheese and bake for a further 20 minutes. Alternatively, defrost completely, scatter with cheese and bake for 25-30 minutes at 200C/ 400F/ GM6