Remove all the bones from the salmon fillet. Mix the salts and the sugar in one bowl and the fennel and dill in a separate bowl. Place the fish in a salting dish, skin side down. Rub the fish all over with the sugar salt mixture. Add the coarsely ground white peppercorns and cover the whole fillet with the fennel-dill mixture. Marinate in the refrigerator for 24 hours.
Put the mustards and the sugar in a bowl and mix well. Add the white wine vinegar and whisk in the oil, a few drops at a time. Season to taste with salt and white pepper.
Remove the fillet from the cold just before serving. Leave the herbs on the surface of the fish. Cut the seasoned fillet diagonally into narrow strips.