Season the flank steak evenly on both sides with the sea salt, black pepper, ground cumin, and garlic powder.
Heat the olive oil in a cast-iron skillet over medium-high heat and sear the steak for 3-4 minutes per side until medium-rare, then transfer to a board to rest.
In the same skillet, add the sliced bell peppers and red onions, sautéing for 5 minutes until they are tender and slightly charred.
Slice the rested steak against the grain into thin strips.
In a small bowl, mash the avocado with lime juice and chopped cilantro to create a fresh guacamole.
Place the tortilla in a clean skillet over medium heat, sprinkle half the cheese on one side, layer on the steak and vegetables, top with the remaining cheese, and fold in half.
Cook for 2 minutes per side until the tortilla is golden brown and the cheese has completely melted, then serve immediately with the guacamole.