POUR dressing over chicken in non-reactive container. Refrigerate 1 to 2 hours to marinate.
DRAIN chicken; discard marinade. Cook chicken, in batches, in skillet on medium heat 4 to 5 min. on each side or until done (165ºF).
FOR each serving: Spread 1 Tbsp. mayo evenly onto 2 bread slices. Cover 1 bread slice with 7 to 8 spinach leaves, 1 pepper half and 1 chicken breast; top with 1 cheese slice and remaining bread slice. Cook in panini maker 5 min. or until cheese is melted and sandwich is golden brown.