01 - In a mixing bowl, toss the thinly sliced steak with olive oil, ground cumin, smoked paprika, garlic powder, salt, black pepper, and fresh lime juice until evenly coated. Let the meat 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 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 and red bell pepper along with the jalapeño if using. Cook for 3 to 4 minutes until the vegetables are softened and lightly caramelized. Remove from the pan and set aside with the steak.
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 empty half of the tortilla over the filling to close it.
05 - Cook the folded quesadilla for 2 to 3 minutes per side, pressing down gently with a spatula, until the tortilla is golden brown and crisp on both sides and the cheese has fully melted throughout.
06 - Remove the finished quesadilla from the pan and repeat the assembly and cooking process with the remaining three tortillas, dividing the cheese, steak, vegetables, and cilantro evenly among them.
07 - Allow each quesadilla to rest for 1 to 2 minutes after cooking so the filling settles, then slice into wedges. Serve immediately while hot, alongside salsa, sour cream, and guacamole for dipping.