In a large skillet, heat 2 tablespoons of cooking oil over medium heat. Add the sliced onion, bell pepper, and mushrooms, and sauté for about 5–7 minutes, or until tender and caramelized.

Once the vegetables are sautéed, add the thinly sliced beef to the skillet. Season with ¼ teaspoon of salt and stir to combine. Cook for an additional 5–7 minutes until the beef is browned and cooked through.

Reduce the heat to low and lay a large tortilla in the skillet. Sprinkle half of the 2 cups of shredded provolone cheese evenly over one half of the tortilla.

Spoon the beef and vegetable mixture over the cheese on the tortilla, then top with the remaining provolone cheese, covering the filling completely.

Increase the heat back to medium and cook the folded quesadilla for 2–3 minutes on each side until crispy and golden-brown.

Remove the quesadilla from the skillet and let it rest for a minute before slicing it into wedges. Serve hot with dipping sauce.