1 1/2lb potatoes, thinly sliced with a mandolin (there is no need to peel them, but you can if you feel the urge)
2 leeks, white and light green parts halved lengthwise and sliced twice as thick as the potatoes
1lb Cod or haddock (or similar meaty white fish), thickly sliced on the diagonal
Salt and freshly ground black pepper to taste
1/2 cup heavy cream
1/2 cup shredded Parmigiano Reggiano or similar hard cheese
Pre-heat the oven to 360F and butter the inside of an oval gratin dish.
Use half of the potato slices to cover the base of the dish (overlap to completely cover it), then season with salt and pepper. Cover the potatoes with half the leeks and then layer the fish on top of the leeks. Now cover the fish with the remaining leeks and finish off with a layer of the remaining potatoes (so from bottom up - potatoes, leeks, fish, leeks, potatoes). Pour the cream evenly over the top and season with pepper.
Bake for 40 mins then baste the top with some of the juices. Cover evenly with the cheese. Return to oven for a further 20 mins till top layer is nicely golden brown (if necessary, finish under the grill for the best color). Remove from the oven and allow to stand for 15 mins before serving.
Serves 4