Slice the flank steak into thin strips and season evenly with sea salt, black pepper, and garlic powder.
Heat the olive oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the steak strips to the skillet in a single layer and sear for 2-3 minutes until browned, then remove the meat and set aside.
In the same skillet, add the sliced bell peppers and red onions, sautéing for 4-5 minutes until they are tender and have developed slight char marks.
Wipe the skillet clean and return it to medium heat, then place the whole grain tortilla in the center.
Spread half of the shredded cheddar cheese over one half of the tortilla, then layer the steak and sautéed vegetables on top.
Sprinkle the remaining cheese over the filling and fold the tortilla in half to close.
Cook for 2 minutes per side, pressing down gently with a spatula, until the tortilla is golden brown and the cheese is fully melted.
Transfer to a cutting board, let rest for one minute, then slice into wedges and garnish with fresh cilantro.