Heat olive oil in a deep saucepan. Add onion, celery and carrot. Cook over medium heat for 5-10 minutes until everything is tender and starting to colour. Add garlic and rosemary and cook for another few minutes until fragrant.
Add lentils, vegetable stock and bay leaves. Simmer over a low heat for about 20-25 minutes, until the lentils are tender but still holding their shape. Keep an eye on them during cooking, stirring occasionally, and add a little more water if necessary to prevent the lentils from drying out or sticking to the pan. You want to end up with lentils and vegetables that are tender, with a little liquid surrounding them - but you don't want soup.
Discard bay leaves before serving. Adjust seasoning with salt and pepper to your taste.
If you're a spice lover, try adding a pinch or two of chilli flakes.
Recipe first published on 24 June 2015. Updated and republished on 26 July 2020.