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Fill a pot with cold water, lemon, onion, celery, salt, and herbs. Add your salmon filet and heat to a medium temperature. Cook your salmon until it is opaque and flakes easily. Season with salt, pepper, and herbs.
Fill a pot with 1 US-quart (950ml) (1 L) of cold water. Either squeeze your lemon into the water or cut it into thin slices to insert. Place your onions, celery, and bay leaf into the pot. Sprinkle thyme, dill, and salt on top to taste. For more flavor, try adding peppercorns, other vegetables like carrots, or cooking liquids like chicken stock and white wine. To avoid overcooking your fish, keep your pot well below boiling, aiming for a temperature around 170 F (76 C). If your water does not fully cover the salmon, add just enough to do so. Do not preheat your pot. Starting cold will help your salmon retain moisture and avoid overheating. Let your salmon simmer until it can be easily pulled apart by a fork and is no longer transparent on the inside. Depending on the exact heat, this could take between 20 and 30 minutes. Poached salmon will not change colors as drastically as grilled or baked salmon, so pay extra attention to the opacity and toughness. Sprinkle your salmon with a small amount of salt, pepper, thyme, and dill. Poached salmon has a naturally clean, light flavor, so avoid excessive seasoning. Squeeze lemon over the top to add a zesty kick.