Baked fish and potatoes and a leek and green peppercorn sauce

It's the return of the fish-plus-spuds-plus-leeks combo! I made this with River Cobbler fillets, but it would work with any firm white fish fillets.

Serves 2

2 large fillets of hake, cod, cobbler
Pinch of sea salt and white pepper to taste

2 baking potatoes
sea salt

For the sauce:
2 small leeks, finely sliced
1 shallot, finely sliced
150ml single cream
1 tsp English mustard
1 tsp green peppercorns
1 splash of rosé wine
Juice of half a lemon
handful of chopped curly parsley
20g unsalted butter

Put the potatoes (in their skins) in a large pan of water and bring back to the boil. Parboil them for ten minutes, then dry them off, rub the skins with sea salt and put them on the rack in an oven pre-heated to 200C. Bake them for about 25 minutes.
Meanwhile, make the sauce: put the butter and the bruised garlic in a pan and turn it to a medium heat. Discard the garlic when it has turned golden brown and flavoured the butter. Add the leeks, shallots and peppercorns and cook for a few minutes, stirring occasionally so the leek doesn't colour. Add the lemon juice and the wine and cover for a minute to let the flavours infuse. Uncover and add the parsley, mustard and cream. Mix together well and cook covered again for a couple of minutes. Once the cream has come to the boil, take off the heat.
Meanwhile, wrap the fish fillets loosely in foil along with a pinch of salt and white pepper and some of the parsley, finely chopped. Bake in the oven with the potatoes for fifteen minutes.
To serve, press down on the top the potato with the edge of a spoon handle or similar so it splits open, lay the fish next to it and spoon the sauce over both.