Hold crab firmly, twist off two claws and legs. Pull body section from shell, discarding feathery gills, the greyish-white stomach sac behind the head and any green matter.
Using a teaspoon, scrape into a bowl all brown meat inside shell. Discard inner membrane attached on either side. Press natural dark line on underside of shell to break along the line neatly; discard the broken inner shell. Wash and dry the main shell.
Snap legs in half by bending backwards at joint. Using a hammer, gently crack all shells; scrape white meat into a second bowl, using a skewer to get into crevices. Discard any bits of shell. Crack large claws, remove meat and add to bowl.
Cut body section in half, pick out white meat from honeycomb structure and add to bowl.
To dress the crab, mix brown meat with lemon juice, mayonnaise and breadcrumbs and season with salt and pepper. Flake white meat with fork. Spoon brown mixture in a line in centre of shell. Carefully place white meat on either side.
Separate hard-boiled egg; sieve yolk and chop white. Cover dark meat with egg yolk, sprinkle a line of egg white on either side of this, then a line of parsley. Garnish with lemon slices and lettuce and serve with mayonnaise.