In a fish poacher or a pot large enough to hold the salmon, combine the stock, onion, celery, peppercorns and dill. Add 3 inches of water and bring to a boil.Add the salmon fillet (it should be fully submerged) and bring the water to a simmer. Cover and simmer gently over low heat until the salmon is just cooked through; about 8 to 10 minutesTurn off the heat and let stand for 5 minutes. Using 2 spatulas, carefully transfer the salmon fillet to a large platter.Dab off any white bits and let stand at room temperature for 15 minutes.Cover loosely with plastic wrap and refrigerate until cool.Make the horseradish and dill mayo while it cools. (refer to video for instructions)Garnish the salmon with lemon slices and dill sprigs, and serve with the horseradish and dill mayo.