Remove the salmon skin and discard. Trim any ragged-looking edges off the fish, and remove the pin bones with tweezers if necessary. Portion into two pieces, and put into a deep, flat heatproof vessel, somewhat larger than the salmon. A deep baking dish is ideal.
Squeeze one half of the lemon into a medium saucepan, and toss the shell in too. Add the other lemon half, the onion, herbs, spices, sugar and salt.
Tip in the vinegar, and add the water. Stir briefly to combine.
Set over a medium heat, and bring to a boil. Cook for 5 minutes, so the flavors absorb into the broth a little.
Carefully pour the hot liquid over the salmon, tucking in the onions, lemons, herbs and spices around the edges. Leave to cool to room temperature then cover and leave in the fridge for at least two hours; overnight is fine.
Carefully lift the fish out of the poaching liquid onto the serving plates and sprinkle with a few sea salt flakes.