Three days before you want to serve the salmon, put it, skin-side down, in a rimmed dish. Combine the salt and sugar, then set aside.
Fry the coriander seeds, ground coriander, caraway seeds and peppercorns in a small dry frying pan set over a medium heat for 2-3 mins. Tip into a spice grinder or small food processor along with the bay, and blitz until finely ground. Stir the spice mixture into the salt and sugar mix. Pat the spiced salt mixture all over and around the salmon so it’s well packed onto the flesh. Wrap tightly in cling film, then set a plate or board and something heavy (such as a couple of cans) on top to weigh the fish down. Chill for 48 hrs.
Heat the paprika, cayenne and black treacle in a pan until loosened, then remove from the heat and leave to cool slightly until just warm. Brush the excess salt off the salmon and rinse the fish under cold running water. Pat dry with kitchen paper, then transfer to a clean dish. Brush with the warm treacle mixture until well coated (you may need to generously brush the fish several times before the coating covers the fish well), then wrap in a fresh sheet of cling film. Chill for another 24 hrs.
When you’re ready to serve, set the salmon on a board and thinly slice with a sharp knife, avoiding the layer of skin at the bottom. Serve with soft cheese, rye bread or bagels, pickles and some chopped dill for everyone to help themselves.