Start by slicing the raw meat perpendicularly into thin slices against a grain.
Prepare your marinade by mixing 1/3 cup olive oil, soy sauce, vinegar, sugar, cumin, and minced garlic together in a bowl.
Transfer your meat slices into the bowl. Cover the bowl with plastic wrap and let it marinate for 30 minutes- 1 hour. You can let it marinate overnight in the fridge.
In a large pan on medium-high heat, add 1 tablespoon oil and then stir fry the meat slices without the extra marinade, for 5 minutes. Remove any moisture formed during the cooking.
In a separate skillet, put one tortilla brushed with oil from both sides. On top of the tortilla, spread shreds of cheese generously and add the carne asada, sliced onions, and some more cheese on top. Cover with another tortilla covered with oil. Let it cook for 1-2 minutes until the bottom is golden brown. Flip the whole Mulitas so that the other tortilla obtain the same color and crust.
To put more toppings, remove with caution the top tortilla and stuff it with mashed avocado, diced tomatoes, and cilantro.
Repeat with the rest of the tortillas. Serve warm and enjoy!