Salmon: Mix the salt, sugar, pepper and chopped dill. Bone the salmon and rub with the salt and sugar mixture. Place one fillet on top of the other with the skin facing out, and place on a bed of the mixture in a deep dish. Cover the dish with aluminum foil, then cover with a board and a weight. Refrigerate for 1-4 days, turning every 24 hours. Scrape the mixture off the fish, and cut the salmon into dice.
Avocados: Peel the avocados and cut into dice the same size as the salmon. Sprinkle with lemon juice and season.
Gelatin: Follow the instructions on the packet to make the gelatin, pour a little over the base of a mold and refrigerate until set. Meanwhile, keep the rest of the gelatin in a cool place allowing it to take the consistency of egg white but without setting. Place the mixed salmon and avocado dice on top of the set gelatin, then pour the rest around the sides and over the top. Refrigerate until set.
To release the gelatin mold, place the mold in hot water for a few seconds, gently pull the gelatin away from the sides of the mold then turn out onto a dish. Serve with the dill sauce.
Dill sauce: Mix the mustard, dill, pepper, salt and sugar with the mayonnaise. Add the cream and a little stock.