For the leek sauceWash and finely chop 80g of leeks and brown them in a casserole dish with the potatoes and 50g of chestnuts, pour on the milk and water, season with salt and pepper and cook over low heat for 30 minutes. When the leeks are cooked, put the vegetables into a blender for a few seconds. In the meantime, in a frying pan, fry the lard, the remaining chestnuts and chopped leeks, add the leek sauce and simmer for 2 minutes.For the fonduePut the diced Fontina cheese into a tall narrow pan, pour on the boiling milk, cook in bain marie for 15 minutes, stirring continually with a wooden spoon. When the cheese has melted, add the egg yolk and sieve into a bowl. Keep the fondue warm in bain marie. Cook the pasta for 11 minutes in salted water. Drain it and pour it into a large frying pan with the leek sauce to unite the two ingredients.To ServePour the fondue over a large serving plate, add the fusilli and garnish with a few fried leeks and a slice of crisp lard. Serve very hot.