Slice the flank steak into thin strips across the grain to ensure a tender bite.
In a small bowl, toss the steak strips with chili powder, cumin, sea salt, black pepper, and the juice from the lime.
Heat avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Add the steak, diced bell peppers, and red onions to the skillet, sautéing for 4-5 minutes until the steak is browned and the vegetables are tender-crisp.
Remove the steak and vegetable mixture from the pan and set aside, then wipe the skillet clean with a paper towel.
Place the tortilla in the skillet over medium heat, sprinkling half of the shredded cheddar on one side.
Layer the steak and veggie mixture over the cheese, top with the remaining cheese, and fold the tortilla in half.
Cook for 2 minutes per side until the tortilla is golden-brown and the cheese is completely melted.
Remove from heat, slice into wedges, and garnish with freshly chopped cilantro before serving.