Slice the flank steak against the grain into very thin strips, then season evenly with sea salt, black pepper, garlic powder, and ground cumin.
Heat a large cast-iron skillet over medium-high heat with half of the avocado oil.
Add the sliced red bell peppers and red onions to the skillet, sautéing for 3-4 minutes until they are tender-crisp and slightly charred. Remove and set aside.
Add the remaining avocado oil to the same skillet and sear the steak strips for 1-2 minutes per side until browned and cooked through. Remove steak from the pan.
Wipe the skillet clean, then place the whole wheat tortilla in the pan over medium heat.
Sprinkle half of the Monterey Jack cheese on one half of the tortilla, then layer the steak and sautéed peppers and onions on top.
Top with the remaining cheese and fold the tortilla in half.
Cook for 2 minutes per side, pressing down slightly, until the tortilla is golden-brown and the cheese is fully melted.
Slice into wedges and serve immediately.