In a saucepan; combine lentils, carrots, garlic, bay leaf and thyme. Add enough water to cover by at least 1 inch. Bring to a boil, then reduce heat and simmer, uncovered, for 15 to 20 minutes or until lentils are tender. Drain, discarding bay leaf.
Transfer lentil mixture to a bowl. Add onion, celery, parsley, oil, lemon juice, salt and pepper. Toss to mix. Let cool to room temperature before serving or keep in the refrigerator once cooled.