Thinly slice the flank steak against the grain into bite-sized strips.
In a small bowl, toss the steak strips with chili powder, cumin, sea salt, black pepper, and the juice from the lime half.
Heat avocado oil in a large skillet over medium-high heat.
Add the sliced onions and bell peppers to the skillet, sautéing for 3-4 minutes until softened and slightly charred.
Push the vegetables to the side of the pan and add the seasoned steak, searing for 2 minutes per side until browned and cooked through.
Remove the steak and vegetable mixture from the pan and set aside, then wipe the skillet clean.
Place the tortilla in the skillet over medium heat, sprinkling half of the cheddar cheese on one side.
Layer the steak and pepper mixture over the cheese, then top with the remaining cheese and fold the tortilla in half.
Cook for 2-3 minutes per side, pressing down slightly, until the tortilla is golden brown and the cheese is completely melted.
Slice the quesadilla into wedges and serve immediately.