Place the lentils in a sieve and rinse under running water. Set aside.
Heat a large pan and add the onions. Stir, now and again, to prevent burning and allow the onions to soften. Add the garlic and keep everything moving in the pan for a few minutes.
Add the thyme and bay leaf and continue to stir for another minute.
Lower the heat and add the leeks and carrots and allow them to “sweat” a little in the pan. Stir occasionally for 4 or 5 minutes.
Tip the lentils into the pan and add 75ml of liter of the water. Bring to a gentle simmer and cook until the lentils are soft. There should be almost no liquid in the bottom of the pan but you can add more water if necessary to prevent the lentils from sticking.
Before serving season with salt and pepper and, if you choose, a good splash of white wine.