Place one half, skin side down, in a large dish or casserole in which it can lie flat. Combine the salt, sugar and peppercorns and rub the flesh of the salmon very well with this mixture. Place the dill on top. Rub the rest of the salt-sugar-peppercorn mixture into the flesh of the second piece of salmon and place over the dill, skin side up, re-forming the salmon shape. Cover salmon with foil, then put a board or a large plate on top and weigh this down with canned goods. Refrigerate for 36 to 48 hours, turning the salmon over each day so that it cures evenly and basting with the liquid that accumulated from the curing process. Each time, weigh it down again. About 4 hours before the fish is done, make the sweet mustard sauce. To make the sauce, put the mustards, sugar, vinegar and oil in a small bowl and beat with a whisk until it has the consistency of a thin mayonnaise (the mustard thickens the liquid). Mix in the dill and refrigerate for 3 to 4 hours before serving to let the flavors mellow. At the end of the curing time, remove the fish from the liquid, scrape away the dill and the seasonings and dry it well in paper towels. To serve, place on a carving board and slice on the diagonal, detaching the flesh from the skin as you do. Serve with the sweet mustard sauce and buttered rye bread.