Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the onion and fry, stirring, for 5 minutes or until pale golden. Add the beetroot to the pan with the drained beans, stock and plenty of salt and pepper. Bring to the boil, stirring.
Transfer the beetroot mixture to the slow cooker pot. Cover with the lid and cook on low for 3½–4½ hours or until the beetroot is tender. Stir well and lift the pot out of the cooker.
Stir the cucumber into the yogurt and season with salt and pepper. Arrange the lettuce leaves on 4–5 individual plates. Top with the warm beetroot salad, then add spoonfuls of the cucumber yogurt. Scatter the spring onions and coriander or mint over the top and serve at once.