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First make the poaching liquid for the fish. Put the peppercorns, 2 tsp salt, vinegar, fennel, bay leaves and parsley stalks in a saucepan with 2.5 litres of water, and bring to the boil. Boil for 10 mins so all the flavours infuse into the liquid.
Make sure that you choose a roasting tin big enough to hold the salmon fillet. Strain the liquid into the empty tin, bring to the boil on the hob, then add the fish, skin-side down, making sure it is covered in liquid. Scatter around the lemon slices, and simmer gently for 1-2 mins until the fish changes colour completely. Cover the surface with a large piece of foil, turn off the heat and leave to cool completely. The fish will cook as the liquid cools.
To make the salsa verde, put all the ingredients into the bowl of a food processor, then blitz to make a bright, slightly chunky sauce. Season to taste.
Carefully remove the salmon to a serving platter and peel away the skin if you like. Serve with the salsa verde, new potatoes and asparagus.