Fillet the salmon, removing the line of small bones that runs half the length of each fillet using a pair of pliers, rinse and dry each fillet. Mix the peppercorns with the sea salt, juniper berries and sugar. Roughly chop
Take a sheet of foil large enough to wrap the salmon in and lay the fillets side by side on the foil. Evenly spread each fillet with the salt and sugar mixture, using it all up, and cover with an even layer of chopped dill, then squeeze the lemon juice over the fillets. Place one fillet on top of the other, fold over the foil and seal it tightly all the way round. Leave to stand in a refrigerator for 36 hours, turning the parcel over every 6-8 hours. After 36 hours, open the parcel, separate the fillets and scrape them clean, but save the juice and use to baste the fillets. Finely chop the reserved dill, spread an even layer over each fillet and baste again with the juice. Lay one fillet on top of the other, reseal the foil bag and leave to stand for further 12 hours, turning again after 6 hours.
To make the sauce, finely chop the leaves of the dill. Mix the egg yolk with the mustard and sugar in a bowl and then gradually whisk in the oil as if making mayonnaise. If the mixture becomes too thick, then thin it down with a little warm water. Add the chopped dill, season to taste with a little salt, the lemon juice and a little more sugar if necessary. If the sauce is still too thick, a little warm water can be added until it is of a good consistency.