In a small bowl, combine the chili powder, cumin, garlic powder, sea salt, and black pepper.
Rub the spice blend evenly onto both sides of the flank steak.
Heat avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the steak for 3-4 minutes per side for medium-rare, then transfer to a cutting board and let rest for 5 minutes.
While the steak rests, mash the avocado in a small bowl with lime juice, finely diced red onion, and chopped cilantro to make the guacamole.
Thinly slice the steak against the grain into bite-sized strips.
Wipe out the skillet and return to medium heat. Place the tortilla in the pan and sprinkle half of the cheddar cheese over one side.
Layer the sliced steak over the cheese, top with the remaining cheese, and fold the tortilla in half.
Cook for 2 minutes per side until the tortilla is golden and crisp and the cheese has melted.
Slice the quesadilla into wedges and serve immediately with the fresh guacamole.