Peel and chop the onions into a small dice. Tip into a large, heavy-based saucepan and pour over the malt vinegar. Bring to a gentle simmer, cover loosely and cook for 15 minutes until softened, stirring once or twice.
While the onions are cooking you can prepare the beans. Trim the ends, then cut down each side to remove any strings. Place each bean flat on the board and slice thinly l