Thinly slice the flank steak against the grain into bite-sized strips to ensure tenderness.
Season the steak strips evenly with cumin, garlic powder, sea salt, and black pepper.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Add the sliced red onion and bell pepper to the skillet, sautéing for 3-4 minutes until they are slightly softened and charred.
Push the vegetables to the side and add the steak strips, searing them for 2-3 minutes until browned and just cooked through.
Transfer the steak and vegetables to a plate and wipe the skillet clean with a paper towel.
Place the tortilla in the skillet, sprinkle the Monterey Jack cheese over one half, and top with the steak and vegetable mixture.
Fold the tortilla in half and toast for 2 minutes per side until the cheese is melted and the exterior is golden and crisp.
While the quesadilla toasts, mash the avocado in a small bowl with lime juice and chopped cilantro until velvety.
Slice the quesadilla into three triangles and serve immediately with the fresh guacamole on the side.