Take aluminum foil and wrap the tortillas in it; place them in the preheated oven for about 15 minutes. Now, prepare a mixture of mashed avocado, lime juice, and salt. To prepare the taco, place a tablespoonful of shredded beef on the warm tortilla, and top it with salsa, avocado mixture, onion, and cilantro. Fold the tortillas and serve to eat.