Preheat oven to 350°
Remove stem and brown gills from undersides of mushrooms using a spoon; discard gills. Place mushrooms, stem side down, on a baking sheet coated with cooking spray. Bake at 350° for 10 minutes; cool.
Heat oil in a large nonstick skillet over medium-high heat. Add onion slices; sauté 10-15 minutes, or until onions start to soften and become caramelized.
Reduce heat and stir in 1 cup Bertolli® Gold Label Imported Balsamic Vinegar with Caramelized Onions, salt and pepper. Heat through and simmer for 10 minutes, stirring occasionally.
In a small dish, combine bread crumbs and parmesan cheese.
Spoon remaining 1/2 cup Bertolli® Gold Label Imported Balsamic Vinegar with Caramelized Onions sauce into the casserole dish. Top with cooked portobello mushrooms, stem side up. Spoon 1/2 cup sauce and caramelized onions mixture into the cap of each mushroom. top with bread crumb mixture and then garnish with fresh parsley.
Bake in a 350° oven for 25 minutes and golden brown. Serve warm.