Scallops: Place the bruised lemongrass in the water of a steamer and heat. Place the whole scallops over the water and steam until just opened; 5-10 minutes.
Remove the scallops and cool. When cool gently remove the scallop meat from the shell with a sharp knife.
Dressing: Place the shallot, chillies, tomatoes, lime juice and fish sauce in a bowl and toss.
To Serve: Place the scallops on a plate and spoon the dressing over the scallops. Arrange the lettuce and cucumbers around the scallops.