One day before you want to serve this, rinse the salmon fillet with cold water and pat it dry with kitchen paper. Blitz the salt, sugar, peppercorns, fennel seeds, coriander seeds and clementine zest in a food processor, then scatter roughly a third of the cure mix across the bottom of plastic tray or container. Sit the salmon skin-side down in the tray, then top with the remaining cure. Cover the fish with cling film and leave in the fridge for about 8 hours, preferably no longer than 12.
Finely chop the cornichons and dill, then stir into the crème fraiche, along with a squeeze of lemon juice. Chill until ready to serve.
Rinse the cure off the salmon with cold water and pat dry with kitchen paper. If you get a chance, the fish will benefit from a couple of hours of extra drying time, left uncovered on a plate in the fridge. Slice thinly, starting from the widest end of the fillet and serve with the watercress, fennel, sliced fruit and dressing.