HEAT oven to 375°F.
REMOVE stems from mushrooms; chop stems. Use spoon or sharp knife to scrape gills from caps; discard gills. Brush 1 Tbsp. dressing onto rounded sides of caps; place, rounded-sides down, in foil-lined 15x10x1-inch pan.
HEAT remaining dressing in large saucepan on medium-high heat. Add chopped stems, peppers and garlic; cook and stir 2 min. Add spinach; cover. Simmer on medium-low heat 4 min. or until spinach is wilted, stirring after 2 min. Spoon over mushroom caps; top with cheese.
BAKE 18 to 20 min. or until mushrooms are tender.