Put one of the salmon fillets, skin-side down, on to a large sheet of cling film. Mix the dill with the salt, sugar and crushed peppercorns and spread it over the salmon. Place the other fillet on top, skin side up.
Tightly wrap in 2 or 3 layers of cling film and lift on to a large shallow tray. Rest a slightly smaller tray or chopping board on top of the fish and weight it down. Refrigerate for 2 days turning the fish every 12 hours so that the briny mixture which has developed inside the parcel, bastes the outside of the fish.
For the horseradish and mustard sauce, stir together all the ingredients except for the cream. Whip the cream into soft peaks, stir in the horseradish mixture, cover and chill.
Remove the fish from the briny mixture and slice it as you would smoked salmon, then serve with the sauce.