Heat the oil in a large pan and cook the onion for 3-4 minutes until soft.
Add the garlic, cumin, coriander and chilli powder and cook for 1 minute until fragrant.
Tip in the red lentils, chopped tomatoes and vegetable stock.
Bring to the boil, then reduce the heat and simmer for 20 minutes until the lentils have broken down completely.
Stir occasionally to prevent the lentils sticking to the bottom.
Season with salt and pepper to taste.
Blend with a stick blender for a smooth soup, or leave chunky if you prefer.
Serve hot with bread for dunking.