HEAT grill to medium-high heat.
TOSS spinach with tomatoes, feta and 2 Tbsp. dressing; reserve half the spinach mixture.
MAKE cut in one long side of each chicken breast, being careful to not cut through to opposite side. Stuff pockets evenly with remaining spinach mixture.
GRILL chicken 5 to 7 min. on each side or until done (165ºF), brushing occasionally with remaining dressing. Serve topped with reserved spinach mixture.