Creamy lentil veggie curry

Name: Creamy lentil & veggie curry
Ingredients: 2 tbsp rapeseed oil; 1 onion, chopped; 1 tsp ground cumin; 1 tbsp Madras curry powder; 200g red lentils; 2 sweet potatoes, peeled and cut into cubes; 1l veg stock; 400g canned peeled cherry tomatoes; 200g green beans, trimmed and cut into short lengths; 4 tbsp Greek yogurt plus more for the top if you like; ½ small bunch coriander, chopped; ¼ cucumber, finely chopped (optional); naan bread and rice to serve