Pick over the lentils and remove any stones etc. Rinse well.
Heat the olive oil in a large pan over medium heat. Once oil is hot, add the onion and carrots. Cook for a few minutes, until the vegetables have started to soften, then add the garlic.
Add the lentils, bay leaf, and water to the pan. Simmer over low-medium heat for 20-30 minutes, until lentils are softened.