Detach the langoustine tails and shell them, keeping the last ring and tail intact. Wash and pat dry the celery leaves.
Make the batter: Pour 17 cl very cold water into a bowl, add the egg yolk, and whisk briskly. Gradually whisk in the 125g flour and salt to form a smooth batter.
Finely chop 2 celery leaves and fold them into the batter.
Heat the oil in a deep pan or fryer. Lightly flour the langoustines with the 3 tablespoons flour, dip in batter, and fry for 2-3 minutes, turning once, until crisp and golden. Fry all langoustines first.
Then fry the celery leaves in the batter for 1 minute.
Lift out with a slotted spoon, drain on paper towels, arrange on a platter, and serve immediately with lemon wedges.
