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 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 along with the 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 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 over to enclose the filling.
05 - Cook the 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 - Repeat the assembly and cooking process with the remaining tortillas, steak, vegetables, and cheese, wiping the pan between each quesadilla if needed.
07 - Allow the quesadillas to rest for 1 to 2 minutes before slicing each into wedges. Serve immediately with salsa, sour cream, and guacamole on the side.