Lentil & sweet potato curry

Name: Lentil & sweet potato curry
Prep time: 10 min
Cook time: 25 min
Total time: 35 min
Cuisine: Indian
Ingredients: 2 tbsp vegetable or olive oil; 1 red onion chopped; 1 tsp cumin seeds; 1 tsp mustard seeds (any colour); 1 tbsp medium curry powder; 100g red or green lentil or a mixture; 2 medium sweet potatoes peeled and cut into chunks; 500ml vegetable stock; 400g can chopped tomato; 400g can chickpea drained; ¼ small pack coriander (optional); natural yogurt and naan bread, to serve