This is luxurious, despite being a one-pot dish. It’s absolutely good enough to cook for friends on a Saturday night.
Prep time: 15 minutes
Cook time: 1 hour 15 minutes
1kg potatoes, peeled
2 large leeks, coarse outer leaves and green ends removed, trimmed and very finely sliced
1 large garlic clove, very finely sliced
525ml double cream
125ml full-fat milk
For the fish
60g fresh white breadcrumbs
1½ tbsp finely chopped parsley
60g mature cheddar, finely grated
20g Parmesan, grated
20g butter, melted
4 thick pieces of cod fillet, about 175g each (getting thick pieces is very important)
wedges of lemon, to serve
Heat the oven to 170C/ 160C fan/gas mark 3½.
Using a mandoline, slice the potatoes very finely, almost wafer thin. As you do this, put them in a round, shallow, flameproof casserole – 30cm in diameter – layering them with the leeks, garlic and some seasoning as you go.
Pour the cream and milk over the vegetables and set the pan over a medium heat. Bring the contents of the dish to the boil, but don’t let the liquid bubble up over the sides. Turn the heat down and cook over a medium heat for 10 minutes. This just helps soften the potatoes before the dish goes into the oven, and raises the temperature so it’s not going in cold.
Put the dish in the oven and cook for 30-35 minutes. The liquid will reduce quite a bit and the potatoes will become tender.
Mix the breadcrumbs, parsley, cheeses, melted butter and some seasoning together. Take the dish out of the oven. Turn the heat up to 190C/180C fan/gas mark 5 and wait until the oven reaches the new temperature.
Put the pieces of fish on top of the potatoes, divide the breadcrumb mixture between them – patting the crust down on top of the fish with a spoon – and return the dish to the oven.
Bake until the fish is cooked through, about 15-16 minutes. Check the flesh in the thickest part – it should be white, not glassy.
The potatoes will be too hot to serve immediately, so leave the dish to cool for about 7-8 minutes – remember that the fish will continue to cook during this time. Squeeze some lemon over the top and serve from the dish in which it has been cooked. Offer lemon wedges on the side.