Slice the flank steak into very thin strips against the grain to ensure tenderness.
In a small bowl, whisk together the lime juice, chili powder, cumin, garlic powder, sea salt, and black pepper. Toss the steak strips in this marinade and let sit for 5 minutes.
Thinly slice the red bell pepper and red onion.
Heat half of the avocado oil in a large cast-iron skillet over medium-high heat. Sauté the peppers and onions for 3-4 minutes until softened and slightly charred, then remove from the pan.
Add the remaining oil to the skillet. Add the steak strips in a single layer and sear for 2 minutes per side until browned and cooked through. Remove steak from the pan.
Wipe the skillet clean and return to medium heat. Place the tortilla in the pan and sprinkle half of the cheese over one half of the tortilla.
Layer the cooked steak and sautéed vegetables over the cheese, then top with the remaining cheese.
Fold the tortilla in half and press down firmly. Cook for 2-3 minutes per side until the tortilla is golden brown and crispy and the cheese is fully melted.
Remove from the pan, let rest for 1 minute, and slice into wedges before serving.