Drizzle 1 tablespoon of olive oil into a wide, deep pan and add the diced onion. Cover with a lid and cook on a low heat for 5 minutes until the onions have softened but are not brown.Add in the garlic, ginger, cumin and cayenne pepper. Lower the heat and fry for 1 more minute. Then add in the tinned tomatoes, lentils, water or stock and some salt and pepper. Bring to the boil, cover with a lid and then turn down and simmer for 30 minutes until the lentils are really soft. Stir occasionally.When the soup is ready, squeeze in the juice of 1 lemon and add more water if you think the soup is looking too thick.Ladle the soup into bowls and serve garnished with fresh coriander.