Pat the salmon dry with kitchen paper and run your hands over the flesh to see if there are any stray small bones – if there are, use a pair of tweezers to pull them out. Set the salmon fillets aside.
Tip the salt, sugar, peppercorns, lemon zest, juniper and dill into a food processor and blitz until you have a bright green, wet salt mixture or ‘cure’. Unravel some cling film but keep it attached to the roll. Lay the first fillet of salmon, skin-side down and then pack the cure over the flesh. Drizzle with gin, if using and top with the 2nd fillet, flesh-side down. Roll the sandwiched fillets tightly in cling film to create a package.
Place the fish in a shallow baking dish or shallow-sided tray and lay another tray on top. Weigh the tray down with a couple a tins or bottles and place in the fridge for at least 48 hrs or up to 4 days turning the fish over every 12 hours or so. The longer you leave it the more cured it will become.
To make the sauce, tip all the sauce ingredients into a blender. Blitz until you have a thickened dressing.
To serve unwrap the fish and brush off the marinade with kitchen paper. Rinse it if you like. You can slice the fish classically into long thin slices, leaving the skin behind or remove the skin it and slice it straight down. Serve the sliced fish on a large platter or individual plates with pumpernickel bread, dill and mustard sauce.