Thinly slice the flank steak against the grain into strips and season evenly with sea salt, black pepper, and ground cumin.
De-seed the poblano pepper and slice it into thin strips along with the red onion.
Heat the avocado oil in a large cast-iron skillet over medium-high heat.
Add the poblano peppers and onions to the skillet, sautéing for 4-5 minutes until they are softened and show light char marks.
Remove the vegetables from the pan and set aside; add the steak strips to the hot skillet in a single layer.
Sear the steak for 2-3 minutes without moving to develop a crust, then toss and cook for another minute until browned.
Wipe the skillet clean and return it to medium heat, then place the tortilla in the center.
Sprinkle half of the Monterey Jack cheese over one half of the tortilla, then layer on the steak and vegetable mixture.
Top with the remaining cheese and fold the tortilla over to close the quesadilla.
Cook for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted.
Remove from heat, garnish with fresh cilantro, and slice into three wedges before serving.