Place the oil in a large frying pan over medium heat. Combine the chicken, breaking it up with a wooden spoon. Add the garlic and cook, stirring, until the chicken is cooked through 4 to 5 minutes. Turn off the heat. Stir in scallions, ginger, hoisin sauce, soy sauce, sesame oil, and water chestnuts. Cook the chicken filling over medium low heat, stirring, until it bubbles and is heated through, 2 to 3 minutes. Keep the filling warm if you are serving it immediately.