Heat the oil in a large soup pot over a medium heat. Add the onion and fry for 5 minutes until softened. Add the garlic and ginger and fry for another couple of minutes.
Add the sweet potato, pepper followed by the harissa, paprika, ras-el-hanout, cumin and cinnamon to the pot. Stir everything together and make sure that everything is coated in the spices.
Tip in the stock, chopped tomatoes and lentils. Bring to the boil and then leave to simmer for about 20 minutes. The lentils should be cooked and the sweet potato should be softened. Add the chickpeas. Check the texture and add a little more stock if you'd like it thinner.
After another five minutes remove the soup from the heat and stir through a squeeze of lemon juice. Serve into bowls and top with a crumbling of feta, a dollop of Greek yoghurt and some chopped coriander.