In a small bowl, combine the ground cumin, garlic powder, chili powder, half of the sea salt, and the black pepper.
Rub the spice mixture evenly over both sides of the flank steak to ensure every inch is seasoned.
Heat the olive oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the skillet and sear for 4-5 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
While the steak rests, mash the avocado in a small bowl with half of the lime juice, the diced red onion, and the remaining sea salt.
Thinly slice the rested steak against the grain into strips to ensure a tender bite.
Wipe out the skillet and return it to medium heat, then place the whole wheat tortilla in the pan.
Sprinkle the Monterey Jack cheese over one half of the tortilla and top with the steak strips and fresh cilantro.
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 three wedges and serve immediately with the fresh guacamole and the remaining lime juice drizzled over the top.