In a saucepan, bring the water to a boil with the lentils and garlic head. Cover and simmer over low heat for about 45 minutes or until the lentils are tender. Season with salt and pepper at the end of cooking. Drain, if necessary. Remove the head of garlic and press with your fingers to remove the garlic purée. Set aside.
In a bowl, combine the oil, vinegar, and mustard. Add the lentils, garlic purée, and the remaining ingredients. Stir to combine. Adjust the seasoning.
Serve with Grilled Pork Tenderloin with Espelette Pepper.