Pat the flank steak dry and season both sides evenly with ground cumin, smoked paprika, sea salt, and black pepper.
Heat the olive oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the steak for 4 to 5 minutes per side for medium-rare, then remove from the pan and let it rest on a cutting board for at least 5 minutes.
While the steak rests, add the sliced poblano peppers and red onions to the same skillet, sautéing until tender and slightly charred.
Thinly slice the steak against the grain into bite-sized strips.
Wipe the skillet clean and place the tortilla in the pan over medium heat.
Sprinkle half of the Monterey Jack cheese on one side of the tortilla, then layer on the sliced steak, sautéed peppers, and onions.
Top with the remaining cheese and fold the tortilla in half.
Cook for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted.
Slice into wedges and serve immediately.