Tip the fresh borlotti beans, if using, into a saucepan. Add enough cold water to cover by about 5 cm (2 inches) and bring to the boil. Skim off any scum that rises to the surface, then reduce the heat to a simmer and cook, uncovered, for 30 minutes or until tender; drain. If using canned beans, simply rinse under cold running water before using, then heat gently in a saucepan over a medium heat for 3 minutes.
Put the oil, garlic and chilli in a large bowl. Stir in the warm beans and the celery and season with salt. Leave the salad to cool to room temperature. You can then cover and store in the refrigerator for up to 1 day.
Leave the beans to return to room temperature before serving if you have prepared them in advance. Stir in the prawns and lemon rind and juice, then gently toss through with the rocket or scatter on top. Serve at once.