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 rest for 10 minutes to absorb the flavors.
02 - Heat a large skillet or grill pan over medium-high heat. Sear the marinated steak slices for 2 minutes per side until nicely browned on the outside but still tender inside. Transfer to a plate and set aside.
03 - In the same skillet, sauté the sliced red onion, red bell pepper strips, and jalapeño (if using) for 3 to 4 minutes until softened and lightly caramelized. Remove from the pan.
04 - Wipe the skillet clean and return 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 cooked steak, sautéed vegetables, and fresh 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 the cheese has fully melted.
06 - Repeat the assembly and cooking process with the remaining tortillas, steak, vegetables, cheese, and cilantro until all four quesadillas are done.
07 - Allow the quesadillas to rest for 1 to 2 minutes so the filling sets, then slice each into wedges. Serve hot alongside salsa, sour cream, and guacamole.