Preheat the oven to 200°C. Heat the oil in a large soup pot over a medium heat. Add the onion and cook for five minutes. Add the sweet potato and garlic and cook for another 5 minutes then add the ras-el-hanout and a good helping of salt and pepper and cook for 2 minutes more.
Pour in the stock and lentils and bring to the boil. Cook for 20 minutes until the potatoes are tender. Blend the soup until smooth then return to the heat and add the coconut milk, check for seasoning and keep warm until ready to serve.
Cut the bread into cubes and place on a baking tray with a drizzle of olive oil. Place in the oven for 8-10 minutes until crispy and brown. Remove from the oven and sprinkle with sumac. Toss.
While the croutons are cooking heat a drizzle of olive oil in a small frying pan over a medium high heat. Add the chorizo and cook until crispy. Serve the soup topped with the croutons, chorizo and a dollop of yoghurt.