Heat the oil in a large saucepan over a medium heat and fry the onion for 5 minutes until softened.
Add the garlic and carrot and cook for 2 more minutes, stirring so the garlic does not burn.
Stir in the red lentils, diced potato, cumin (if using), salt and pepper.
Pour in the vegetable stock and bring to the boil, then reduce the heat and simmer for 20 minutes until the lentils and potatoes are tender.
Blend half the soup with a stick blender or transfer half to a blender and back, leaving it partly smooth and partly chunky.
Taste and adjust the seasoning, then serve hot with bread if you have it.