Two days before serving: Cook lobster. Crack shells and claws. Mix together all the marinade ingredients in a separate jug or bowl and pour over the lobster. Leave in the refrigerator overnight.
Next day: Cook the asparagus in boiling, salted water until just tender. Refresh in cold water and drain. Then cut it into 1/2-inch dice and reserve. Chop the cucumber into pieces the same size as the asparagus. Make the anchovy mayonnaise: put all the ingredients except the olive oil and hot water in a blender or food processor and whisk together. Add the olive oil; drop by drop, continuing to blend and gradually increasing the addition of the oil to a trickle. Then add the hot water until you have a thick, smooth mayonnaise (it is important to add the oil slowly and steadily in this way, rather than all at once, or you may run the risk of the mixture curdling). Season with pepper to taste. Reserve.
Just before serving, assemble the Tartare: Remove the lobster from the marinade, cut it into cubes of a similar size to the asparagus and cucumber and combine all three together in a bowl. Add the anchovy mayonnaise and season with pepper (the anchovies will probably make the mixture salty enough). Mix thoroughly. To serve, either pile the mixture into shallow bowls or plates of for a neater presentation, place a 2 1/2-inch diameter pastry cutter in the center of each bowl or plate. Pack each ring with the lobster and asparagus mixture, pressing down as you do so. Neaten off the top, then carefully lift off the pastry rings. Garnish each serving of tartare with a good sprig of chervil and serve with the bread.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.