Use metal tongs to hold the onion over hot coals or char in a hot griddle pan until just blackened all over. Leave to cool completely, then peel away the outer blackened layer and finely chop the rest of the onion. You can also simply finely chop the onion without charring it first, if you prefer.

Cook the beans in a pan of boiling salted water for 4-5 mins until just cooked, with a slight snap. Drain and tip into a bowl with the onion, cannellini beans, olive oil and lemon juice. Season well, toss everything together and set aside until needed. This can be prepared several hours ahead and chilled. Stir in the herbs just before serving.