HEAT oven to 400ºF.
PLACE fish in shallow baking dish; drizzle with 3 Tbsp. dressing. Bake 10 to 12 min. or until fish flakes easily with fork.
MEANWHILE, heat remaining dressing in large skillet on medium heat. Add garlic; cook and stir 1 min. Stir in spinach; cover. Cook 2 min.; uncover. Cook 30 sec. or just until spinach is wilted, turning frequently with tongs.
PLACE spinach on 2 serving plates; sprinkle evenly with 2 tsp. cheese. Top with fish and dressing reserved from bottom of baking dish. Sprinkle with remaining cheese.