Season the steak with salt, pepper, and taco seasoning. Sear in a hot skillet, then let it rest for 10 minutes before slicing.
In the same skillet, sauté peppers, onions, garlic, oregano, and red pepper flakes until tender.
Place a slice of cheese in the skillet, top with a tortilla, and cook until the cheese melts and turns golden, about 30–60 seconds. Transfer to a plate, cheese-side up.
Top the cheesy tortilla with sliced steak, sautéed peppers and onions, and mashed avocado. Fold in half.
Serve immediately with fresh salsa or quesadilla sauce.