Slice the flank steak into thin, bite-sized strips against the grain to ensure tenderness.
In a small bowl, toss the steak strips with chili powder, cumin, garlic powder, sea salt, and black pepper until evenly coated.
Heat avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the seasoned steak strips to the skillet and sear for 2-3 minutes until browned and cooked through; remove steak from the pan and set aside.
In the same skillet, sauté sliced bell peppers and onions for 3-4 minutes until softened and slightly charred.
Wipe the skillet clean and place the tortilla inside over medium heat.
Sprinkle half of the shredded cheese on one side of the tortilla, top with the cooked steak and sautéed vegetables, then add the remaining cheese.
Fold the tortilla in half and cook for 2 minutes per side until the exterior is golden brown and the cheese is fully melted.
Squeeze fresh lime juice over the filling before slicing into wedges and serving.