1) Wash salmon and pat dry. Mix to combine salt and sugar. Lay 2 long pieces of plastic wrap on top of each other to form a cross. Layer half the bunch of coriander in the middle of the plastic wrap, add half the salt-sugar mix and press salmon into the mixture, covering the bottom well. Top with the salt-sugar mix, rubbing it in well and place the remaining coriander from the bunch on top. Wrap salmon tightly in plastic wrap and refrigerate 24 hours.
2) Wash off salmon and pat dry. Slice into 1/2 cm cubes and place in a bowl, tossing with chopped coriander, tomatoes, spring onion, and lime juice. Serve on crackers.