Slice the flank steak into very thin strips across the grain for maximum tenderness.
In a small bowl, whisk together the chili powder, sea salt, black pepper, and the juice from the lime.
Toss the steak strips in the lime-spice mixture until every piece is thoroughly coated.
Heat the avocado oil in a large non-stick skillet over medium-high heat until shimmering.
Add the steak to the pan and sear for 2-3 minutes until a dark, flavorful crust forms.
Add the sliced peppers and onions to the skillet, sautéing for another 3 minutes until they are tender-crisp.
Transfer the steak and vegetable mixture to a clean plate and wipe the skillet out with a paper towel.
Place the tortilla in the skillet and sprinkle half of the shredded cheese over one half of the tortilla.
Layer the steak and vegetable mixture over the cheese, then top with the remaining cheese.
Fold the tortilla over and cook for 2 minutes per side, pressing down gently, until the shell is golden-brown and the cheese is fully melted.