To make the caramelized onion + mushroom filling:
Preheat a large skillet on medium heat. Add the butter and olive oil. When the butter is completely melted, add the onions. Let the onions cook down for 5 minutes. Add the sugar and let the onions continue to cook for 15 more minutes. Stir periodically allowing the onions to brown but not burn. Add the garlic, thyme, and Worcestershire sauce and let cook for another 2 minutes. Add the mushrooms. Stir and mix everything together until well incorporated. Continue to cook for another 10 minutes. Add the black pepper, red pepper flakes, and flour. Cook for 1 minute. Add the mushroom broth (or beef broth) in 1 tablespoon at a time. Wait until the onion/mushroom mixture absorbs the liquid before adding the next tablespoon. Once the 4 tablespoons are added, turn off stove remove from heat and add the parsley.

To make the panini:
To assemble the sandwich, place a halved slice of cheese on a piece of bread. Add desired amount of filling and top with another halved slice of cheese and another slice of bread. Spray one side of the sandwich with cooking spray and place that side down on the panini press. Spray the other side and press sandwich for 3 - 4 minutes of until the preferred toast level is achieved.