Defrost the marinated flank steak overnight.
Heat pan over high heat. Add ½ teaspoon avocado oil.
Place the steak in the pan, reserving the marinade that is left in the bag. Sear for 3-4 minutes and then flip. Sear for another 3-4 minutes.
Once the steak has a golden crust on each side, remove from the pan and let rest.
Slice the steak against the grain into strips, or to the size of your liking.
Heat the reserved marinade up and begin to reduce over medium heat, once it is reduced by about half, add the sliced steak back into the pan and coat in the sauce. You can continue to cook the steak until it is cooked to your liking.
Turn off the flame and set aside.
To assemble the quesadillas, place 6 (6-inch) flour tortillas on a flat surface. On one half of each tortilla, divide the Oaxaca cheese among the 6 tortillas, then top with steak, cilantro, mozzarella and cotija cheese.
Heat a large pan over medium-high heat and add ½ teaspoon oil. Fold the tortilla in half and place as many as you can in the pan. Place something heavy on top so they stay closed (I used a smaller cast-iron pan)
Cook for 3-4 minutes on each side until golden crust forms on the tortillas and the cheese is melted.
Serve immediately with your choice of toppings.