01 - In a mixing bowl, toss steak slices with olive oil, ground cumin, smoked paprika, garlic powder, salt, black pepper, and lime juice. Let marinate for 10 minutes.
02 - Heat a large skillet or grill pan over medium-high heat. Sear the steak for 2 minutes per side until browned but still tender. Remove from pan and set aside.
03 - In the same pan, sauté the red onion, red bell pepper, and jalapeño (if using) for 3 to 4 minutes until softened and lightly charred. Remove and set aside.
04 - Wipe the skillet clean and place over medium heat. Lay a flour tortilla flat in the skillet. Sprinkle one quarter of the shredded cheese over half the tortilla, then layer the cooked steak, sautéed vegetables, and fresh cilantro on top of the cheese. 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 the cheese is fully melted.
06 - Repeat the assembly and cooking process with the remaining tortillas, steak, vegetables, cheese, and cilantro.
07 - Allow the quesadillas to rest briefly, then slice each into wedges. Serve hot with salsa, sour cream, and guacamole on the side.