Peel and roughly chop the onions and sweet potatoes - can be chunky.
Combine with the stock or water, lentils, garlic, cumin, cayenne and turmeric in a pot.
Cover and bring to the boil. Simmer until the potatoes and lentils are cooked - about 20-30 minutes.
Puree the soup, adding in the coriander and the juice of the lemon. Add salt and pepper to taste (lots of pepper for me).