Heat the borlotti beans in a saucepan over a medium heat for 3 minutes, adding the measurement water if starting to stick to the base.
Put the oil, garlic and chilli in a large bowl. Stir in the celery, onion and hot beans and season with salt and pepper. Cover and leave to marinate at room temperature for at least 30 minutes and up to 4 hours.
Stir in the tuna and lemon rind and juice. Gently toss in the rocket leaves, taste and adjust the seasoning with extra salt, pepper and lemon juice, if necessary.