Peel and finely grate the garlic and ginger, trim and slice the spring onion, pick and finely chop the coriander leaves, then add to a bowl with the rest of the cocktail sauce ingredients. Mix well to combine, then chill in the fridge for 1 hour.
Trim the lettuce, separate the leaves and divide between plates. Drape over the smoked salmon, then arrange the prawns on top.
Dress the lettuce and seafood generously with the cocktail sauce. Cut the lime into wedges and squeeze a little juice over each plate, drizzle over some oil and finish with a good grinding of black pepper.