01 - In a mixing bowl, combine the sliced steak with olive oil, ground cumin, smoked paprika, garlic powder, salt, black pepper, and 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 nicely browned but still tender. Transfer to a plate and set aside.
03 - In the same pan, sauté the sliced red onion and bell pepper (add jalapeño if using) for 3 to 4 minutes until softened and lightly charred. Remove from the pan and set aside with the steak.
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 steak, sautéed vegetables, and chopped cilantro.
05 - Fold the tortilla over the filling to form a half-moon shape. Press down gently with a spatula and cook for 2 to 3 minutes per side until the tortilla is golden brown and the cheese has fully melted.
06 - Repeat the assembly and cooking process with the remaining tortillas and filling ingredients. Let each quesadilla rest for 1 to 2 minutes before slicing into wedges. Serve hot alongside salsa, sour cream, and guacamole.