01 - In a mixing bowl, combine the thinly sliced steak with olive oil, ground cumin, smoked paprika, garlic powder, salt, black pepper, and fresh lime juice. Toss until evenly coated and let marinate for 10 minutes at room temperature.
02 - Heat a large skillet or grill pan over medium-high heat. Sear the marinated steak slices for 2 minutes per side until a golden-brown crust forms and the meat is still tender. Transfer to a plate and set aside.
03 - In the same skillet, sauté the sliced red onion and bell pepper strips along with the jalapeño if using. Cook for 3 to 4 minutes, stirring occasionally, until the vegetables are softened and lightly charred.
04 - Wipe the skillet clean and return it to medium heat. Lay a flour tortilla flat in the pan. Sprinkle one quarter of the shredded cheese over one half of the tortilla, then layer on a portion of the seared steak, sautéed vegetables, and chopped cilantro. Fold the tortilla over to enclose the filling.
05 - Cook the folded quesadilla for 2 to 3 minutes per side, pressing down lightly with a spatula, until the tortilla is golden brown and crispy and the cheese is fully melted.
06 - Remove the finished quesadilla and repeat the assembly and cooking process with the remaining tortillas, cheese, steak, and vegetables.
07 - Allow each quesadilla to rest for 1 to 2 minutes before slicing into wedges. Serve hot alongside salsa, sour cream, and guacamole.