Combine the lentils and garlic in simmering salted water for 10 minutes, or until the lentils are crisp-tender. Drain and run the lentils under cold water. Discard the garlic.
In a bowl, toss together the lentils, celery, onion, and parsley. In a small bowl, whisk together the lemon juice, olive oil, and water. Drizzle over the lentils and stir gently to incorporate. Season with salt and pepper.