Cut the spring onions into rings. Wash shrimp and king prawns and blanch for 3 minutes in boiling water, remove, let cool. Dip the tomatoes briefly in boiling water, then peel, core and cut into small cubes. Cut the chives into rolls. Mix together lemon juice or vinegar, salt, pepper, mustard and sugar or honey. Then stir in the oil and pour over the tomatoes, spring onions and chives, mix.
Arrange the lettuce leaves in 4 cocktail glasses, place the shrimp and prawns on top. Finally pour the marinade over it and garnish with lemon wedges.