Thinly slice the flank steak against the grain into bite-sized strips to ensure tenderness.
In a medium bowl, toss the steak strips with the olive oil, sea salt, black pepper, ground cumin, and garlic powder until evenly coated.
Heat a cast-iron skillet over medium-high heat and sear the steak for 3 to 4 minutes until deeply browned and caramelized. Remove the steak from the pan and set aside.
Wipe the skillet clean and place one corn tortilla in the pan over medium heat.
Layer half of the shredded cheddar, the cooked steak strips, and the remaining cheese onto the tortilla, then top with the second tortilla.
Cook for 2 minutes per side until the tortillas are golden and crisp and the cheese has fully melted.
While the quesadilla toasts, mash the avocado in a small bowl with lime juice, minced red onion, and chopped cilantro to create a fresh guacamole.
Slice the quesadilla into four wedges and serve immediately with the guacamole on the side.