Rinse lentils well, and simmer them gently to reduce splitting. Check for doneness after about 18 minutes.
To prepare vinaigrette, whisk together all ingredients. Add salt and black pepper to taste.
To prepare salad, combine 4 cups water, lentils, and bay leaves in a medium saucepan; bring to a boil. Reduce heat; simmer 18 to 20 minutes. Drain; discard bay leaves. Gently stir together lentils, celery, onion, parsley, and salt. Add tomatoes and 2 tablespoons chives. Drizzle with vinaigrette; toss gently to coat. Divide watercress evenly among 6 plates; top with 3/4 cup lentil mixture. Top with remaining 2 tablespoons chives.