Rub the flank steak with chili powder, cumin, garlic powder, half the sea salt, and black pepper.
Heat half the olive oil in a cast-iron skillet over medium-high heat and sear the steak for 4-5 minutes per side until medium-rare, then set aside to rest.
In the same skillet, add the remaining oil, sliced bell peppers, and onions, sautéing until they are tender and slightly charred.
While the vegetables cook, mash the avocado in a small bowl with lime juice, chopped cilantro, and the remaining sea salt.
Thinly slice the rested steak against the grain into bite-sized strips.
Wipe the skillet clean and place the tortilla inside over medium heat; sprinkle cheese on one half, then layer the steak and sautéed vegetables on top.
Fold the tortilla in half and cook for 2 minutes per side until the exterior is golden and the cheese is fully melted.
Slice the quesadilla into wedges and serve immediately with the fresh guacamole on the side.