Rub the flank steak evenly with the cumin, chili powder, garlic powder, sea salt, and black pepper.
Heat half of 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.
Remove the steak from the pan to rest for 5 minutes, then sauté the sliced peppers and onions in the same pan until softened and slightly charred.
Thinly slice the rested steak against the grain into bite-sized strips.
Wipe the skillet clean, add the remaining oil, and place the tortilla in the pan over medium heat.
Layer one half of the tortilla with the cheese, sliced steak, and sautéed vegetables, then fold the tortilla over.
Cook for 2 minutes per side until the tortilla is golden and the cheese has completely melted.
In a small bowl, mash the avocado with lime juice and fresh cilantro to create a simple guacamole.
Slice the quesadilla into three wedges and serve immediately with the fresh guacamole on the side.