Bring the potatoes to boil in a medium sized saucepan and then simmer slowly for 30 minutes until cooked. Drain and dry slightly in the pan before mashing or putting through a potato ricer.
Saute the leek and kale in half the butter until soft. Add the spices and cook for a further 2 minutes.
Mix together the potato, vegetables, mustard, egg yolk and crème fraiche in a large bowl, and then form into 4 large cakes (or 8 small ones).
Heat the remaining butter in a large non-stick frying pan and gently cook the potato cakes on both sides until dark golden and crispy.
Top the cakes with the cheese and place under a hot grill until melting. Serve with a shredded white cabbage and carrot salad (or as a side dish with veggie sausages).
