Preheat the oven to 180°C. Place each beetroot on a piece of foil, generously drizzle with olive oil, season with salt and pepper and rub in the oil. Wrap up and roast for 1 hour until tender. Remove from the foil and allow to cool for 15 minutes. Once cool enough to handle, peel and cut into wedges. Combine the beetroot, onion, butter beans and mint in a large bowl. Add the yoghurt, lemon juice and a few pinches of salt and pepper and stir to combine. Spoon into a large serving bowl and garnish with the extra mint leaves.