Cook the potatoes in a large saucepan of boiling water for 12–15 minutes until tender, adding the green beans for the last 3 minutes. Drain and refresh under cold running water.
Place the cannellini beans, olives and onion in a large bowl and stir in the potatoes and green beans.
Whisk together the oil, lemon rind and juice, sugar and salt and pepper in a jug, then stir in the chopped herbs. Pour over the bean and potato mixture and toss well before serving.