1. Whisk dressing ingredients together in a bowl until the sugar has completely dissolved.
2. In a medium bowl,gently toss tuna,cucumber,spring onions,shallots,coriander and season with salt to taste.
3. Place lettuce on 4 plates and divide the tuna mix among them,making a mound on each bed of lettuce. (Alternatively,allow everyone to make their own parcels,san choi bao style.) Sprinkle with the sesame seeds.
Note: Serve withpork belly and kimchi stir fry,steamed rice and steamed Asian greens drizzled with oyster sauce.