Make layers of leek, potato, onion and celery in an ovenproof dish.
Place the lamb thighs on top and coat with olive oil, salt and pepper.
Roast in the oven at 160ºC for 2 hours.
If the juices run clear, take it out and set it aside.
For the coating:
Mix the mint, sugar, cashews, walnuts and the egg.
Top the lamb thighs with the mixture and place it under the grill under it forms a crust.