Rub the flank steak evenly with the cumin, garlic powder, sea salt, and black pepper on both sides.
Heat the avocado oil in a heavy cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for approximately 4 minutes per side for a perfect medium-rare finish.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing thinly against the grain into bite-sized strips.
While the steak rests, mash the avocado in a small bowl with the lime juice, diced red onion, and chopped cilantro until creamy.
Wipe out the skillet and place the whole wheat tortilla in the pan over medium heat to warm through.
Sprinkle the Monterey Jack cheese over one half of the tortilla, layer the sliced steak on top, and fold the tortilla over.
Cook for 2 minutes per side until the tortilla is golden brown and the cheese is completely melted.
Slice the quesadilla into wedges and serve immediately with the fresh guacamole on the side.