Season the flank steak evenly on both sides with sea salt, black pepper, garlic powder, and cumin.
Heat avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the steak for 3-4 minutes per side until it reaches your desired level of doneness, then remove from the pan and let rest for 5 minutes.
In the same skillet, sauté the sliced red onion and bell peppers for 3-5 minutes until tender and slightly charred.
Thinly slice the rested steak against the grain into bite-sized strips.
In a small bowl, mash the avocado with lime juice and a pinch of salt to create the guacamole.
Place the tortilla in a clean skillet over medium heat, sprinkle cheese over one half, layer on the steak and peppers, then fold the tortilla.
Toast for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted.
Slice into wedges and serve immediately with the fresh guacamole and cilantro.