Ingredients
2 large potatoes, washed 
3 tsp olive oil 
150g rock salt 
25g butter 
1 leek, thinly sliced 
100g Cheddar, grated 
10g parmesan, very finely grated 
Salt and pepper 

Method
Preheat the oven to 200°C. Place the salt on a baking tray and rub the potatoes with the olive oil and season. Roast the potatoes for roughly 1 hour - 1 hour 30 minutes or until cooked through and the skin crispy. (The exact time here will depend on the size of your potato.  When it is ready you will be able to put a skewer through easily despite the crispy skin.) 

Meanwhile cook the leeks in a little of the butter and a tsp of oil slowly until soft and a little darker than golden.  Keep to one side. 

When the potatoes are soft cut in half and very carefully scoop out all of the soft potato taking care not to damage the skin, it is sometimes easier to leave a little layer of potato to keep the skin from collapsing. 

Mash the soft potato and put through the rest of the butter and a good pinch or so of salt.  Mix through the leeks, cheese and a good grinding of black pepepr.  Taste to check the seasoning and adjust where necessary.   

Pile the potato mix back into the skin and top with the grated parmesan and bake for a further 15 minutes.