1. Rinse the lentils and remove any foreign matter.
2. Transfer the lentils to a large saucepan then cover with a copious amount of water, which should cover the lentils by at least 3-4 inches. Add the bay leaf and thyme.
3. Bring to a boil, then reduce heat, add a bit of salt, and simmer for 20-25 minutes, until the lentils are just tender, adding more water if necessary. Be sure not to overcook them.
4. While the lentils are cooking, heat a few spoonfuls of olive oil in a skillet and add the carrots, onions, and fennel (if using). Sprinkle with salt and pepper, and cook, stirring frequently until tender. Set aside.
5. In a large bowl, mix together the ingredients for the vinaigrette.
6. When the lentils are done, drain them well, then toss them in the vinaigrette with the cooked vegetables. Stir a few times to release the steam. Taste, and season with more salt, pepper, and olive oil if desired. Remove bay leave and thyme sprigs.