Thinly slice the flank steak against the grain into bite-sized strips.
In a small bowl, combine the cumin, chili powder, garlic powder, sea salt, and black pepper.
Toss the steak strips with half of the spice blend and the lime juice to marinate briefly.
Heat the olive oil in a cast-iron skillet over medium-high heat and sear the steak until deeply browned, about 3-4 minutes.
Add the sliced red bell pepper and red onion to the skillet, sautéing until the vegetables are tender-crisp and slightly charred.
In a separate bowl, mash the avocado with the remaining spice blend and cilantro to create a smooth guacamole.
Place the tortilla in a clean pan over medium heat, sprinkle with Monterey Jack cheese, and top with the steak and vegetable mixture.
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.