Ideally, you would want two 500 gram (1 lb) skin on fillets from the center cut of the fillet, remove the pin bones if that is not already done. But you can use the smaller fillets from Tesco, as what is in this photo. When I make this, I will use two of these fillets on the bottom, and two on top, just reduce the salt and sugar in equal proportions.
Mix together the salt, sugars, and fennel seeds. Sprinkle some of the mixture along the bottom of a glass baking dish, sprinkle some dill on that. Place half of the fillets skin side down on that.
Sprinkle half the remaining salt mixture on the fish and half of the remaining dill. Lay the second piece or pieces of salmon on top of the salmon already in the dish, skin side up.
Sprinkle remaining salt mixture and dill over the top of the fillet. Cover the dish with plastic wrap. Set another smaller dish on top of the fillets and weight it down with some canned goods, a brick wrapped in foil, or any kind of heavy weight.
Place in the fridge for two days to cure, turn the fillets after one day, so the top piece becomes the bottom piece, this is to cure them evenly.
After two days, remove from the fridge and remove from the dish. Rinse the fillet very well with cold water, and use your fingers to rub off all the salt and dill. Dry the fillets with paper towels.
Place a fillet skin side down on your cutting board, hold your knife at a angle and slice as thin as you can down to the skin but not through the skin. Repeat with remaining fillet or fillets.
Place the thin slices on a serving tray, sprinkle with purple onion, dill, and capers. Serve with toasted rye bread and lemon wedges.