DO-AHEAD TIP: Marinate chicken for at least 30 minutes (see recipe).
Place chicken in a large zipper-topped plastic bag. In a small bowl, whisk together mustard, lemon juice, olive oil, parsley, sugar, salt and pepper.
Reserve about 4 tablespoons of this mixture in a cup for a later use.
Stir in garlic and water to remaining marinade in the bowl. Pour mixture over chicken then seal the bag and smoosh the chicken around to thoroughly coat.
Refrigerate for at least 30 minutes, turning once to keep chicken coated.
Preheat grill or oven broiler. Lightly oil grill grate/broiler pan. Grill/broil chicken for 5 to 7 minutes per side.
To serve, drizzle the reserved marinade over grilled/broiled chicken. Enjoy!!!