1. Completely shell the tails of eight langoustines.
2. Devein them, place them flat on paper towels and refrigerate for the carpaccio.
3. Shell the tails of the remaining langoustines, leaving on the 3 tail fins; devein and keep them in a circular arc by piercing each tail with a toothpick.
4. Empty four of the heads and cut them open with scissors to use as garnish. Blanch for one minute and refresh in cold water.

Preparing the sea urchins

1. Open the sea urchins with scissors by cutting off the tops, remove the tongues with an espresso spoon, rinse them gently under a drizzle of cold water and set aside in a cool place.
2. Wash the sea urchin shells out thoroughly; set aside.
3. Wash the shellfish (cockles and mussels) and brush if necessary.
4. Set the fennel leaves aside in a cool place.

Preparing the cockles and mussels

1. Remove the outer layer from the fennel and thinly slice the rest of the bulb. Do the same with the shallot.
2. In a covered pan, cook the garlic, fennel, shallot and thyme with the white wine for 10 minutes. Add the cockles and mussels with a little water or fish stock. Cook until the shellfish open.
3. Remove the shellfish from the pan and remove the meat from the shells. Beard the mussels and remove the sand pockets from the cockles.
4. Strain the liquid, add the saffron and adjust the seasoning. There should be just enough to fill the sea urchins: approximately 200 ml (3/4 cup).
5. Add the gelatin which has soaked in ice water for 10 minutes.
6. With an apple corer, cut out a cylinder from the potatoes approximately 1.5 cm in diameter. Cut 12 pieces, each 1 cm long. Hollow out the centre slightly. Cook in salted water and drain.

Preparation one hour before serving

1. Into each sea urchin shell place one-quarter of the cockles and mussels and one tongue, pour the cold saffron gelatin mixture over top and let set 1 hour. On top arrange the rest of the tongues and the fennel leaves.
2. Cut the langoustines into very thin slices like carpaccio and place them on a sheet of paper greased with olive oil. With a brush, also place some oil on the langoustines, then divide among the cold plates.
3. Add a little fleur de sel (fine sea salt) and the sifted pepper. Keep cold.

Presentation

1. Season the potatoes with a little olive oil so they shine.
2. Divide the caviar in the centre of the plates.
3. On a small plate place a sea urchin, the potatoes with caviar and the langoustine garnish. Quickly sear the remaining 8 seasoned langoustines in a skillet, for a maximum of 45 seconds, remove the toothpicks and place them while still warm onto the plates. Serve immediately.