CUT chicken breasts horizontally in half; pound to 1/4-inch thickness. Combine tomatoes and 3 Tbsp. dressing.
HEAT oil in large nonstick skillet on medium-high heat. Add chicken; cook 2 to 3 min. on each side or until done (165°F), drizzling with remaining dressing for the last minute. Transfer to platter; cover to keep warm.
ADD tomato mixture to skillet; bring just to boil, stirring frequently. Add spinach; cook on medium heat 2 to 3 min. or just until spinach starts to wilt, stirring constantly.
SPOON spinach mixture over chicken. Serve with rice.
