Slice the flank steak into very thin strips against the grain and place in a bowl.
Toss the steak with chili powder, cumin, sea salt, black pepper, and lime juice until evenly coated.
Thinly slice the red bell pepper and yellow onion into strips.
Heat avocado oil in a large cast-iron skillet over medium-high heat.
Add the peppers and onions to the skillet, sautéing for 3-4 minutes until tender-crisp with slight char marks.
Add the seasoned steak to the skillet and sear for 2-3 minutes, stirring occasionally, until browned and cooked through.
Remove the steak and vegetable mixture from the skillet and set aside on a plate; wipe the skillet clean.
Place the tortilla in the warm skillet and sprinkle half of the cheddar cheese over one half of the tortilla.
Layer the steak and vegetable mixture over the cheese, then top with the remaining cheese and fresh cilantro.
Fold the tortilla in half and press down firmly with a spatula.
Cook for 2 minutes per side until the tortilla is golden brown and crispy and the cheese is fully melted.
Remove from the heat, let rest for one minute, then slice into three wedges and serve.