01 - In a mixing bowl, toss the sliced steak with olive oil, ground cumin, smoked paprika, garlic powder, salt, black pepper, and lime juice until evenly coated. 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 nicely browned but still tender. Transfer to a plate and set aside.
03 - In the same pan, sauté the sliced onion and bell pepper along with the jalapeño if using. Cook for 3 to 4 minutes until the vegetables are softened and lightly charred. 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 a quarter of the shredded cheese over one half of the tortilla, then layer on a portion of the seared 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 crispy and the cheese is fully melted.
06 - Remove the finished quesadilla and repeat the assembly and cooking process with the remaining tortillas, steak, vegetables, cheese, and cilantro.
07 - Allow each quesadilla to rest for 1 to 2 minutes before slicing into wedges. Serve immediately with salsa, sour cream, and guacamole on the side.