Slice the flank steak into very thin strips across the grain and toss them in a small bowl with the chili powder, cumin, sea salt, and black pepper.
Heat the avocado oil in a large cast-iron skillet or non-stick pan over medium-high heat until shimmering.
Add the steak strips to the hot pan and sear for 3-4 minutes until browned and slightly caramelized, then transfer the meat to a plate.
In the same pan, add the sliced red bell peppers and red onions, sautéing for 3 minutes until they are tender-crisp with a slight char.
Return the steak to the pan, squeeze the juice of half a lime over the mixture, and toss everything together for 30 seconds before removing from heat.
Wipe the skillet clean and place the tortilla in the center; sprinkle half of the cheese over one side of the tortilla.
Layer the steak and veggie mixture over the cheese, top with the remaining cheese, and fold the tortilla in half.
Cook for 2 minutes per side, pressing down gently with a spatula, until the tortilla is golden brown and the cheese is fully melted.