Slice the flank steak into very thin strips against the grain and toss them in a bowl with the chili powder, cumin, sea salt, and black pepper.
Heat the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Add the steak strips to the hot skillet and sear for 2-3 minutes until browned and caramelized, then remove the meat and set aside.
In the same skillet, add the sliced red bell peppers and yellow onions, sautéing for 3-4 minutes until tender-crisp.
Return the steak to the skillet, stir in the fresh lime juice and cilantro to combine, then transfer the entire mixture to a plate.
Wipe the skillet clean and place the tortilla in the center, sprinkling half of the cheddar cheese over one side.
Top the cheese with the steak and pepper mixture, sprinkle the remaining cheese on top, and fold the tortilla in half.
Press down firmly with a spatula and cook for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted.