Put the prawns into a shallow dish. Squeeze over the juice of 2 lemons, toss them around a little so the lemon juice coats them, then leave them to marinate for 15–20 minutes until pink all over. Drain off the juice.
Mix the chilli with the cabbage, add your finely grated lemon zest, then add the marinated prawns, a splash of olive oil and the juice of your remaining lemon. Toss gently together with the chervil, season with salt and freshly ground black pepper and arrange on your plates.