Thinly slice the flank steak against the grain into bite-sized strips.
In a small bowl, toss the steak strips with chili powder, cumin, garlic powder, sea salt, black pepper, and lime juice until well coated.
Heat the avocado oil in a large non-stick skillet over medium-high heat.
Add the sliced bell peppers and red onions to the skillet, sautéing for 3-4 minutes until they are tender and slightly charred.
Push the vegetables to the side of the pan and add the steak strips. Sear the steak for 2-3 minutes until browned and cooked through, then mix with the vegetables and remove everything from the pan.
Wipe the skillet clean and return it to medium heat. Place the corn tortillas in the pan.
Sprinkle half of the Monterey Jack cheese onto one side of each tortilla, top with the steak and pepper mixture, and add the remaining cheese.
Fold the tortillas in half and press down firmly. Cook for 2 minutes per side until the tortillas are golden and crispy and the cheese is fully melted.
Remove from the heat, garnish with fresh cilantro, and slice into wedges before serving.