Slice the flank steak into very thin strips against the grain and place in a bowl.
Toss the steak with the ground cumin, chili powder, sea salt, and black pepper until evenly coated.
Heat the olive oil in a large cast-iron skillet over medium-high heat.
Add the steak to the skillet and sear for 3-4 minutes until browned and slightly charred, then remove from the pan.
In the same skillet, add the sliced bell peppers and red onions, sautéing for 3 minutes until tender-crisp.
In a small bowl, mash the avocado with the lime juice and a pinch of salt to create the guacamole.
Wipe out the skillet and place the tortilla inside over medium heat.
Sprinkle the cheese over one half of the tortilla, then layer on the cooked steak, peppers, and onions.
Fold the tortilla in half and toast 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 and chopped cilantro.