Bring a pan of water to the boil. Add the marron and cook for 2 minutes. Take out and remove tails from heads. Place heads back in, trying to leave the open side facing up to avoid taking on too much water. Cook for a further 4 minutes. Remove claws and take meat from the shells.
Gently take the flesh from the marron tails, using scissors to cut the underside of the shell and crack open. Slice into 1cm thick pieces and place in a bowl. Zest in one finger lime and add its flesh. Squeeze in half a lemon and add a pinch of salt. Mix well and set aside to cure.
Place the egg yolks, mustard and remaining lemon juice in a bowl. Whisk whilst adding a slow stream of vegetable oil to form a mayonnaise. Once thick, add the head flesh of the marron and whisk through. Place all prepared and remaining ingredients randomly into a large flat plate. The more random the presentation, the better.