Pod the beans, cover them with water, bring to the boil and skim. Turn the heat down to a simmer, then add the garlic, bay and chilli, and simmer until the beans are fully cooked, about 40 minutes. When they are cooked, set aside in their liquor.
Prepare the vegetables, cutting them into very fine dice. Make the gremolata, using the zest of the lemon, then juice the lemon.
For the dressing, in a serving bowl heap the celery, carrot and onion dice, the lemon juice and olive oil. Season a little. Using a slotted spoon carefully drain half the beans. Add to the bowl while still hot. Toss and taste, adjust the seasoning, then add the gremolata. Serve the same day, do not refrigerate. Store the remainder of the beans in their liquor in the fridge when cold.