Put fish into a shallow bowl, skin down, pull out remaining bones with a pair of tweezers.
Wash about 2/3 of the dill, then chop it finely (stalks and all). Coarsely grind peppercorns (pestle & mortar). Mix pepper, salt, sugar and sprinkle over the fish, then cover with dill.
Now the fish has to be weighed down with something heavy so it'll absorb the seasoning properly. You could first cover the fish with cling film, put a cutting board and a few heavy stones on top, for example. Put into the fridge and leave to marinade for at least 24 h. Check every now and then for liquid at the bottom of the bowl (spoon over the fish).
After 24 h, the fish is ready to be cut up into very thin slices: lift out of bowl, hold knife at a 45° angle (roughly) and cut the meat off the skin.
Prepare the mustard sauce: pour the marinade/juice that has accumulated in the bowl through a sieve, then add mustard, 1/2 teaspoon sugar, some salt & pepper and the white wine vinegar. Slowly pour in the oil while stirring the sauce. Now chop the rest of the dill (without the stalks) and add it to the mixture.