Cook the beans in boiling salted water for 8-10 minutes. Stone the olives. Peel and chop onion. Wash and quarter the tomatoes and remove the seeds. Cut the flesh into cubes.
Place the olives, onion and tomatoes in the mixing bowl of a food processor. Add lemon juice and mustard and chop everything into small pieces, but not too fine. Finally mix in olive oil and season with salt and pepper.
Drain the beans and arrange on plates with the tapenade.