Season the flank steak evenly on both sides with the ground cumin, garlic powder, sea salt, and black pepper.
Heat the olive oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the skillet and sear for 3 to 4 minutes per side for medium-rare, then transfer to a cutting board to rest.
In the same skillet, sauté the sliced red onion and bell pepper for 4 minutes until they are tender and slightly charred.
Thinly slice the rested steak against the grain into bite-sized strips to ensure maximum tenderness.
Wipe the skillet clean, place the tortilla inside, and layer one half with the cheese, steak strips, and sautéed vegetables.
Fold the tortilla over and cook for 2 minutes per side until the exterior is golden and the cheese has fully melted.
In a small bowl, mash the avocado with the lime juice and chopped cilantro, then serve the sliced quesadilla with the fresh guacamole.