30 minutes or fewer
If you use 10 to 12 small portobellos instead of 6 large ones, this dish could be served as an appetizer. For a flavor twist, substitute marinated artichoke hearts for the sun-dried tomatoes.
Preheat broiler. Spray baking sheet with nonstick cooking spray.
Prepare polenta according to package directions. Remove from heat, and stir in tomatoes, spinach and Parmesan cheese; add salt and pepper to taste if desired.
Scrape off brown gills from inside mushroom caps using spoon. Stir 2 Tbs. gills into polenta mixture, and discard remaining gills. Brush both sides of mushroom caps with reserved oil from tomatoes. Place caps, stem side down, on baking sheet, and broil 5 minutes on each side, or until soft.
Spoon polenta mixture into mushroom caps. Broil mushrooms about 3 minutes, or until golden brown. Using spatula, transfer mushrooms to serving plates.