Lay the salmon fillet skin side down on a sheet of cling film. Warm the treacle in a bowl over a pan of simmering water until it is just runny. Mix the fennel seeds, lemon zest, salt, mustard, pepper and treacle together. Spread evenly over the salmon and wrap well in more cling film. Place on a tray, still skin side down, and leave at room temperature for 1 hour, then refrigerate for 48 hours.