Peel and finely chop the onion. Melt the butter in a large saucepan over low heat, add the onion and fry gently for 5 minutes to soften but not brown. Meanwhile, peel and cut the carrots and potatoes in even-sized chunks. Add to the pan and toss the vegetables in the hot butter.
Add the lentils to the pan along with the stock and bay leaf and season with salt and pepper. Bring to the bo