Pat the top sirloin steak dry with a paper towel and slice it into thin, bite-sized strips against the grain.
In a small bowl, combine the sea salt, black pepper, garlic powder, and cumin, then toss the steak strips in the seasoning until evenly coated.
Heat the avocado 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 and caramelized, then remove from the pan and set aside.
In the same skillet, add the sliced bell peppers and red onion, sautéing for 4-5 minutes until they are tender and slightly charred.
Wipe the skillet clean and 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 on the cooked steak, peppers, and onions.
Top with the remaining cheese and fold the tortilla over to close the quesadilla.
Cook for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted.
Remove from the heat, let rest for one minute, and slice into wedges before serving.