Few things in life are better than sweet caramelized onions. Add cheese and mushrooms and you have a fancy and festive Thanksgiving appetizer the whole family will enjoy! Simply caramelize an onion, sauté chopped spinach, and stuff the mixture into fresh portobello mushrooms then bake! Then top each mushroom with Parmesan and mozzarella cheeses, and bake again until melted and golden brown. These Caramelized Onion and Spinach Stuffed Mushrooms topped with gooey cheese is sure to be a Thanksgiving hit!