Heat the oil in a heavy-based saucepan. Add the garlic and onions and cook them over a medium heat for 6-7 minutes until they turn a rich brown colour, stirring frequently to prevent them sticking.
Add the lentils and cook for a further 1-2 minutes. Then add the stock, raise the heat and bring the soup to the boil. Reduce the heat, cover and simmer for 15-20 minutes until the lentils are almost soft.
Place a nonstick frying pan over a high heat, add the coriander and cumin and dry-fry them for 1-2 minutes until the aroma rises, then add them to the soup.
Raise the heat under the soup, add the lemon juice and lemon slices and season to taste, then let it simmer for 5 minutes. Sprinkle the soup with a little black pepper and serve.