Sweet potato & chard aloo

1 tbsp olive oil
1 onion, finely chopped
200g pack Swiss chard, leaves and stems separated and shredded
3 cloves garlic, finely chopped
20g piece ginger, peeled and finely chopped
4 sweet potatoes, peeled and cut into chunks
1 tbsp medium curry powder
200g fat-free natural yogurt
2 tbsp mango chutney
1-2 jalapeño green chillies, finely chopped (deseeded if liked)
½ x 28g pack coriander, shredded