Put the shredded lettuce in two small bowls, drizzle with a little olive oil and squeeze over a ¼ of a lemon over each. Season with salt and pepper.
Mix together the tomato purée, mayonnaise, shallot and finely chopped herbs, season to taste. Fold the crab into the mayonnaise, try not to break it up too much. Pile on top of the shredded lettuce.
Place a lemon wedge on the side of each crab cocktail to squeeze over and serve with buttered brown bread.