Slice the flank steak into very thin strips against the grain and toss in a bowl with chili powder, cumin, sea salt, black pepper, and lime juice.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the sliced bell peppers and onions to the skillet, sautéing for 3-4 minutes until softened and slightly charred, then remove them from the pan and set aside.
Add the steak strips to the same hot skillet in a single layer and sear for 2-3 minutes without moving to develop a crust, then toss and cook for 1 more minute until browned.
Remove the steak from the pan and wipe the skillet clean with a paper towel.
Place the tortilla in the skillet over medium heat and sprinkle half of the Monterey Jack cheese over one half of the tortilla.
Layer the cooked steak and sautéed vegetables over the cheese, then top with the remaining cheese and fold the tortilla in half.
Cook for 2 minutes per side, pressing down slightly with a spatula, until the tortilla is golden and crispy and the cheese is fully melted.
Transfer to a cutting board, slice into three wedges, and garnish with fresh chopped cilantro before serving.