Skate, potato, apple and spinach gratin with apple crème fraîche
800g skate, removed from both sides of the cartilage to give approximately 375g 600g potatoes
2 Cox's apples, peeled and cored
2tsp raw apple cider vinegar 200g young leaf spinach 100g full-fat crème fraîche 20g salted butter, plus extra for buttering the dish
Cut the skate into strips, season with salt and freshly ground black pepper and set aside. Put the cartilage in a pan, season with salt and cover with 1.25 litres of water. Bring to a simmer for around 10 minutes.
Using a mandolin, thinly slice the potatoes and put into cold water. Halve and slice the apple in the same way, then place in apple cider vinegar. Toss to coat.
Put the spinach in a bowl with a good pinch of sea salt and pour over boiling water to cover. Leave for 3 minutes, refresh under cold water and drain well. Squeeze out as much liquid as possible and set aside.
Lift the cartilage from the water with a slotted spoon and discard (uncooked, this can be frozen and added to other bones and prawn shells to make fish stock). Add the potatoes, cook over a medium heat for about 4 minutes until heated through then transfer from the water into a colander. Add apple to the pan and cook for a few minutes, then drain through a colander set over a bowl to collect the liquid. Lift out a third of the apple and purée in a mini-blender. When cold, add to the crème fraîche and chill. Put the liquid back in the pan and boil for 10 minutes until reduced a little. Preheat the oven to 200C/400F/Gas 6 and butter the gratin dish.
Layer up the ingredients starting with a third of the potatoes. Squeeze the spinach again, chop roughly and add half to the potato layer. Add half the apple slices and half the fish. Repeat with the same proportions, then finish the top with the final third of potato.
Spoon over a few tablespoons of the reduced cooking liquid. Top with knobs of butter and grind over a little freshly ground black pepper. Cook for about 20 minutes, then turn the oven onto a grill setting and cook for a few more minutes (keep a close eye on it) until golden. Serve with the apple crème fraîche.