Thinly slice the sirloin steak against the grain into bite-sized strips.
In a small bowl, combine the cumin, garlic powder, sea salt, and black pepper, then toss with the steak strips to coat evenly.
Heat the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Add the steak strips to the hot skillet and sear for 2-3 minutes until a golden-brown crust forms; remove the steak and set aside.
In the same skillet, add the sliced bell peppers and onions, sautéing for 3-4 minutes until tender-crisp and slightly charred.
Wipe the skillet clean and place the tortilla in the center over medium heat.
Spread half of the Monterey Jack cheese on one half of the tortilla, then layer on the seared steak and sautéed vegetables.
Top with the remaining cheese and fold the tortilla in half.
Cook for 2 minutes per side until the tortilla is toasted and the cheese is fully melted.
Slice into wedges and serve immediately.