With tweezers or pliers, pull out the intermuscular bones of salmon. In a non-reactive dish (glass, porcelain, enamel or stainless) large enough to accommodate the salmon without crowding, scatter half of the sliced onions and sprinkle half of the dill on the onions. In a small mixing bowl combine the salt, sugar, peppercorns and juniper berries and mix them well. Rub half of this mixture into the flesh side of the salmon and spoon rub half the juice and vanilla on the flesh; set the salmon, flesh side down on the dill and onions. Rub remaining salt mixture into skin side of salmon and remaining lemon juice mixture over that. Lay remaining dill on salmon, then cover with last of onion slices. Cover with plastic wrap, then weigh the salmon down with bricks, small iron skillets or canned goods. Cover the dish and weights with aluminum and refrigerate for 2 to 3 days. During the marination time, spoon the juices the salmon give off and marinade back over the salmon. After 2 to 3 days, or when ready to serve, remove salmon from the marinade and scrape off dill and onions from skin side, but leave it on for garnish from flesh side. Slice salmon as thinly as possible on diagonal toward tail and serve with mustard sauce. Or set slices of salmon with a dab of mustard sauce on black bread for hors d'oeuvres.
To make sauce, mix two mustards, sugar and vinegar until pasty. With a wire whisk, slowly beat in oil until it forms a thick mayonnaise like mixture. Stir in chopped dill.