In a large skillet, heat olive oil over medium-high heat. Add the sliced onions, bell peppers, and mushrooms. Sauté until they are tender and slightly caramelized, about 5-7 minutes. Season with salt and pepper to taste. Remove from the skillet and set aside.

In the same skillet, add more olive oil if needed and sear the thinly sliced flank steak on high heat until browned and cooked to your liking, about 3-4 minutes. Season with salt and pepper. Remove from heat and set aside.

To assemble the quesadillas, place a tortilla in the skillet over medium heat. On one half of the tortilla, layer some of the sautéed vegetables, seared steak, and shredded provolone cheese.

Fold the other half of the tortilla over the filling. Cook until the cheese is melted and the tortilla is golden and crispy, about 3-4 minutes per side.

Once cooked, remove from the skillet and repeat with the remaining tortillas and filling.

Cut into wedges and serve hot with your favorite dipping sauce, such as salsa or ranch dressing.