Slice the flank steak into very thin strips and toss in a bowl with the lime juice, cumin, chili powder, sea salt, and black pepper.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the steak strips to the skillet in a single layer and sear for 3-4 minutes until browned and slightly charred, then remove from the pan and set aside.
In the same skillet, add the sliced bell peppers and red onions, sautéing for 3-5 minutes until tender and caramelized.
Wipe the skillet clean and place the tortilla in the center, sprinkling half of the cheddar cheese over one side.
Layer the seared steak and sautéed vegetables over the cheese, top with the remaining cheese, and fold the tortilla in half.
Cook for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted.
In a small bowl, mash the avocado with a pinch of salt and the fresh cilantro to create a quick guacamole.
Slice the quesadilla into wedges and serve immediately with the fresh guacamole on the side.