Halve the avocados.
Remove the stones.
For the guacamole, scoop out the pulp from 1 avocado and place in a bowl. Cut the lemons in half and squeeze the juice. Add 1 tbsp lemon juice to the avocado in the bowl.
Blitz with a hand blender to a creamy mixture. Season with salt and lemon juice.
Peel the remaining avocado halves.
Or lift the peel with a small knife on one side and pull it off.
Cut 2 avocado halves into slices.
Enlarge the central well of 4 halves with a spoon a little.
Brush the halves with lemon juice.
Drain the crab and mix with the yogurt, mayonnaise, salt, pepper, chervil and corn. Taste and use to fill the hollowed avocados.
Cut out small balls from the remaining 2 halves with a melon baller. Dice the remaining avocado.
Place the avocado slices, cubes and balls on plates. Drizzle with some lemon juice so that they do not brown.