Tear half the dill into pieces on to a plate. Add the peppercorns, salt and sugar and mix together. Coat both sides of the salmon in the dill mixture then transfer to a large strong plastic bag with any mixture from the plate. Seal the bag and stand in a shallow dish. Cover with a chopping board and weigh down with an unopened bag of flour, bags of sugar or cans. Marinate in the refrigerator for 2–3 days, turning twice a day.
On the day of serving, make the sauce by mixing the mustard and sugar together in a small bowl. Gradually trickle in the oil, little by little, whisking continuously until thickened. Thin the sauce with vinegar to taste. Stir in the chopped dill and season with pepper.
When ready to serve, lift the salmon out of the marinade, drain, put on a chopping board. Sprinkle with the remaining dill, torn into pieces. Cut into thin slices with the knife at a 45 degree angle to the fish. Arrange on serving plates with spoonfuls of the sauce and a little salad garnish. Serve with brown bread.