Slice the flank steak into very thin strips against the grain to ensure tenderness.
In a small bowl, toss the steak strips with chili powder, cumin, lime juice, sea salt, and black pepper.
Heat the avocado oil in a large skillet over medium-high heat until shimmering.
Add the sliced bell peppers and red onions to the skillet, sautéing for 3 to 4 minutes until softened and slightly charred.
Push the vegetables to the side of the pan and add the steak strips, searing for 2 minutes per side until browned.
Remove the steak and vegetables from the skillet and set aside on a plate.
Wipe the skillet clean and return it to medium heat.
Place the tortilla in the skillet, sprinkle half the cheese on one side, then layer the steak and vegetables on top.
Add the remaining cheese over the filling and fold the tortilla in half to close.
Cook for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted before slicing into wedges.