Season the flank steak evenly on both sides with sea salt, black pepper, garlic powder, and cumin.
Heat the olive oil in a cast-iron skillet over medium-high heat until shimmering.
Place the steak in the hot skillet and sear for approximately 3 to 4 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
In the same skillet, add the sliced red bell peppers and onions, sautéing for 5 minutes until they are softened and slightly charred.
Slice the rested steak into thin strips against the grain to ensure maximum tenderness.
In a small bowl, mash the avocado with the lime juice and a small amount of sea salt until smooth and creamy.
Wipe the skillet clean and place the tortilla inside over medium heat; sprinkle the Monterey Jack cheese over one half.
Layer the sliced steak and sautéed vegetables over the cheese, fold the tortilla in half, and cook for 2 minutes per side until the cheese is melted and the shell is crisp.
Slice the quesadilla into wedges and serve immediately with the fresh guacamole and a garnish of cilantro.