Season the flank steak evenly on both sides with sea salt, black pepper, and garlic powder.
Heat half of the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Place the steak in the skillet and sear for 3 to 4 minutes per side for medium-rare, then remove from heat and let rest on a cutting board.
In the same skillet, add the remaining oil along with the sliced red bell peppers and yellow onions, sautéing until softened and slightly charred.
Thinly slice the rested steak against the grain into bite-sized strips.
Wipe the skillet clean and return it to medium heat, then place the whole wheat tortilla in the pan.
Spread the Monterey Jack cheese over one half of the tortilla, then layer the sliced steak and sautéed vegetables on top.
Fold the tortilla over the filling and cook for 2 minutes per side until the exterior is golden brown and the cheese is fully melted.
Remove from the pan, garnish with fresh cilantro, and slice into wedges before serving.