Heat the oil in a large pan and fry the onion for a few mins until softened. 

Add the spices and cook for 1 min, then stir in the lentils, sweet potatoes, stock and the cherry tomatoes. 

Bring to the boil, then cover and simmer for 20 mins until the lentils and sweet potatoes are tender. 

Add the beans and cook for 2 mins then stir in the yogurt and some seasoning. 

Sprinkle over the coriander and chopped cucumber if using and serve with naan bread and rice.