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 while remaining 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 have softened and developed light char marks. 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 and sprinkle one quarter of the shredded cheese over one half. Layer the seared steak, sautéed vegetables, and chopped cilantro over the cheese, then fold the tortilla in half to enclose the filling.
05 - Cook the quesadilla for 2 to 3 minutes per side, pressing down lightly with a spatula, until the tortilla turns golden brown and crispy and the cheese has fully melted.
06 - Repeat the assembly and cooking process with the remaining tortillas, steak, vegetables, and cheese. Allow each quesadilla to rest for 1 to 2 minutes before slicing into wedges. Serve hot alongside salsa, sour cream, and guacamole.