First make the asparagus mousse
Soak the gelatine in a shallow dish of ice-cold water to soften. Meanwhile, place a large pan over a medium heat and add the butter. When melted and starting to bubble, add the asparagus and cook for about 3 minutes until it softens and starts to collapse. Pour in the cream, bring to a simmer and cook for 3 minutes. Season with salt to taste. Squeeze out the excess water from the gelatine then add it to the asparagus mixture. Immediately tip the contents of the pan into a blender and blend for 2 minutes until smooth. Transfer to a jug. Carefully pour the mixture into 6 individual serving dishes and place in the fridge to set.
To cook and prepare the crab
Bring a large pan of water (big enough to hold the crab fully submerged) to the boil. Season the water generously with salt, to make it as salty as sea water. Once it comes to a rolling boil, lower the crab into the water and cook for 14 minutes.
Carefully lift the crab out of the pan,place on a tray and leave until cool enough to handle. Remove all the legs and claws from the cooked crab, by twisting them away from the body. Now, holding the crab in both hands, use your thumbs to push the body up and out of the hard, top shell or carapace. Remove and discard the dead man’s fingers, stomach sac and hard membranes from the body shell.
Using a spoon, remove the brown crab from the carapace and place in a bowl (you won’t need it all for this dish, so save to eat on toast or freeze it).
Cut the body in half, using a sharp knife, to reveal the little channels of white crab meat. Use a crab pick or the handle of a spoon to pick out all the crab meat from these crevices and put it into a separate bowl.
Using a heavy knife, break the claws with one hard tap if possible and pick out the white meat, prising out the cartilage from the middle of the claw. Do the same with the legs to extract the meat. Once you have extracted all the white meat, with clean hands, pick through it to search for any shell or cartilage. Refrigerate if preparing ahead or set aside while you make the mayonnaise.
To make the mayonnaise
Place the egg yolks, brown crab meat and lemon juice in a bowl and whisk to combine. Slowly add the olive oil in a thin, steady stream, whisking constantly until it is all incorporated. Season with salt and pepper to taste. Add enough of the mayonnaise to the white crab meat to combine.
For the asparagus salad
Carefully slice the asparagus spears lengthways on a mandoline. Add the chopped chervil and dress well with the agrodolce vinegar and olive oil, but don’t overdo it (the dressing will soften the asparagus).
To assemble and serve
Take the asparagus mousses (and crab meat if chilled) out of the fridge 20 minutes before serving. Top each mousse with a pile of crab meat. Arrange the shaved asparagus salad on top and serve straight away.