Begin by thinly slicing the flank steak against the grain into bite-sized strips.
In a bowl, combine the steak strips with lime juice, chili powder, cumin, garlic powder, salt, and pepper. Let marinate for at least 15 minutes.
While the steak marinates, thinly slice the bell peppers and red onion.
Heat a non-stick skillet over medium-high heat. Sauté the bell peppers and onion until they are slightly softened, about 3-4 minutes. Remove from skillet and set aside.
In the same skillet, cook the marinated steak strips until browned and cooked through, about 4-5 minutes.
Assemble the quesadillas by placing one tortilla on a clean skillet over medium heat. Evenly distribute the cooked steak, sautéed vegetables, and sprinkle the shredded cheese over the top. Cover with the second tortilla.
Cook for approximately 2-3 minutes on each side until the tortillas are golden and the cheese has melted.
Remove from heat, let sit for a minute, then slice the quesadilla into wedges.
Serve warm and enjoy the spicy, zesty flavors.