Add the lentils to a small saucepan and cover with at least 1 1/2-inches of water. Add the bay leaf and set the pan over high heat. Bring to a boil, then reduce the heat to medium low and simmer for 20-25 minutes, until tender. Drain, and discard the bay leaf.
Heat the olive oil in a large skillet set over medium heat. Add the onion, carrot and celery. Cook, stirring frequently, until tender (and a little golden brown in places). Remove from heat.
Add the cooked lentils and vegetables to a serving bowl. Set aside.
In a small bowl, combine the vinaigrette ingredients. Whisk thoroughly to combine. Pour over the lentils and vegetables and toss well. Season with salt and pepper, as desired.
Enjoy warm or cool.