Freeze the salmon for 30 minutes. This makes it easier to cube. Using a sharp, long knife, slice into 1 cm or ½ inch cubes.
Grate the ginger and stir it into the lime juice.
Place the salmon cubes in a mixing bowl and pour over the lime juice and ginger dressing. Gently fold with a wooden spoon to ensure the fish is coated well. Chill for ½ hour so the salmon can "cook" in the lime juice.
Dry-roast the sesame seeds in a pan on a low heat until golden.
Cube the cucumber and slice the spring onions. After the salmon has been in the fridge for ½ hour, add the cucumber, onions, coriander and oil into the bowl. Season to taste with salt and pepper and fold gently with the wooden spoon to mix.
Arrange the salmon ceviche in lettuce leaves and scatter over the toastes sesame seeds.
