Put one of the salmon fillets, skin side down, onto a large sheet of clingfilm. Mix the dill with the salt, sugar and crushed peppercorns and spread it over the cut face of the salmon. Place the other fillet on top, skin side up.

Tightly wrap the fish in two or three layers of clingfilm and lift it onto a large, shallow tray. Rest a slightly smaller tray or chopping board on top of the fish and weigh it down. Chill for 2 days, turning the fish every 12 hours so the briny mixture that will develop inside the parcel bastes the fish.

To make the horseradish and mustard sauce, stir together all the ingredients except the cream. Whip the cream into soft peaks, stir in the horseradish mixture, cover and chill.

To serve, remove the fish from the briny mixture and slice it very thinly, as you would smoked salmon. Arrange a few slices of the gravlax on each plate and serve with some of the sauce.