Combine the water, lemon juice, salt and onions in a fish poacher, over medium heat. Bring the liquid to a simmer. Add the fish, covered and cook for about 1 hour or until the fish is flaky. Carefully remove the salmon, discarding the liquid and place on a large platter and chill completely. Using a thin knife, remove the top skin from the salmon. (start removing the skin from below the head and to the tail. In a mixing bowl, mash the potatoes until smooth. Stir in 1/2 to 3/4 cup of the mayonnaise, mustard, onions, parsley and pickles. Mix well. Season with salt an pepper. Set aside. Place the thin slices of cucumber over the exposed salmon meat. Place the potato salad in a pastry bag with a star tip and pipe the salad around the edges of the salmon. Garnish with fresh parsley sprigs. Serve with the remaining mayonnaise and bread.