2: Melt 30 g butter in a frying pan on medium heat. When good and hot add the chopped shallot. Salt and pepper, then cook for 1 minute without colouring.
3: Add the sliced leeks and mix, but no more salt.
4: Cook the leeks, stirring frequently, until just done, but still nice and green. Salt only at the end of cooking. Transfer to a plate (don't leave in the pan or they will continue cooking).
5: Reheat 750 g potato purée if it has been made in advance, then add the leeks. Mix well and check the seasoning. Your Welsh-style potato purée is ready.