PREHEAT grill to medium heat. Brush chicken with 2 Tbsp. of the mustard. Grill or broil 6 to 8 min. on each side or until cooked through (170°F).
MEANWHIILE, drain oranges, reserving 2 Tbsp. of the liquid. Place reserved liquid in medium bowl. Add remaining 2 Tbsp. mustard, the oil, vinegar and ginger; beat with wire whisk until well blended.
COVER large platter with spinach. Cut chicken diagonally into thin slices; arrange over spinach. Top with oranges; drizzle with dressing.