Juice the lemons, scrape pulp and deseed, setting the pulp and juice aside separately.
For the citronette, chop the potatoes into 1cm cubes. Add to a pan of boiling water and until completely soft, then transfer to a blender. Add the milk and lemon pulp, and blitz thoroughly. Slowly drizzle in the olive oil until emulsified.
To make the mayonnaise, in a food processor blend the egg yolk with the vinegar and 1 tsp of lemon juice. Once combined, slowly drizzle in the oil until emulsified.
Steam the langoustines for 4 minutes or blanch in boiling water.
To serve, place three langoustines on each plate. Drizzle over the citronette, sprinkle over chopped parsley and finish with lemon zest. Serve with the mayonnaise on the side.