Every French Onion Soup lover's dream! Caramelized onions, melted cheese, dripping with onion soup.
Preheat the oven to 375 degrees and place the rack in the top third of the oven.
Pull off the mushroom stems. Chop the stems into small pieces and set the mushroom caps aside.
In a medium skillet, melt 3 tablespoons of butter and the olive oil over medium heat. Add onions, chopped mushroom stems, garlic, salt and pepper and sauté for 15 to 20 minutes, stirring occasionally, until very soft and reduced to about one third of original volume.
Add the beef broth, Worcestershire sauce, hot sauce and balsamic vinegar. Cook for an additional 5 minutes, or until liquid is cooked down. Remove from heat and set aside.
Place mushroom caps in an oven-proof dish lined with parchment paper or coated with cooking spray. Turn the mushrooms dome side up and bake for 5 minutes at 375 degrees. Remove from oven and turn oven up to 425 degrees.
Fill the mushrooms with sautéed onion and mushroom mixture, about 1 heaping tablespoon per mushroom.
Bake uncovered for 10 minutes at 425 degrees. Remove from oven and sprinkle mushrooms with cheese. Turn on broiler and cook just until the cheese starts to bubble and turn golden brown. Watch carefully! Top with minced parsley and serve immediately.