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 large cast-iron skillet over medium-high heat until it begins to shimmer.
Add the steak strips to the hot skillet in a single layer, searing for 2-3 minutes until deeply browned and caramelized, then transfer to a plate.
In the same skillet, add the thinly sliced red bell peppers and onions, sautéing for 3-4 minutes until they are tender and slightly charred.
Wipe the skillet clean with a paper towel and return to medium heat.
Place one tortilla in the skillet and sprinkle with half of the shredded cheddar cheese.
Top the cheese with the seared steak and sautéed vegetables, then sprinkle the remaining cheese over the filling and cover with the second tortilla.
Cook for 2-3 minutes per side, pressing down gently with a spatula, until the tortillas are crispy and the cheese is completely melted.
Remove from the heat and let rest for one minute before slicing into wedges.
Garnish with a squeeze of fresh lime juice and chopped cilantro before serving.