Slice the flank steak into thin, bite-sized strips across the grain to ensure tenderness.
In a small bowl, toss the steak strips with the chili powder, cumin, garlic powder, sea salt, and black pepper until evenly coated.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Add the seasoned steak to the skillet and sear for 2-3 minutes until browned and slightly charred; remove the steak and set it aside.
In the same skillet, sauté the sliced bell peppers and onions for 3-4 minutes until they are tender-crisp and slightly caramelized.
Wipe the skillet clean and place the whole wheat tortilla inside over medium heat.
Sprinkle half of the cheddar cheese on one side of the tortilla, then layer on the cooked steak and sautéed vegetables.
Squeeze the juice from the lime over the filling and top with the remaining cheese before folding the tortilla in half.
Cook for 2 minutes per side, pressing down gently, until the exterior is golden brown and the cheese is fully melted.
Slice the quesadilla into three wedges and serve immediately while hot.