Place the dried shredded beef in a bowl and pour warm soaking water over it to cover; let it soak for 20–30 minutes until pliable, then drain and reserve about 1/2 cup of the soaking liquid.
Heat 1–2 tablespoons of oil or lard in a large skillet over medium heat.
Add the diced onion and cook until translucent, about 3–4 minutes, then stir in the minced garlic and finely chopped serrano chile and cook another 1–2 minutes until fragrant.
Add the diced tomato and cook until it breaks down and releases juices, about 4–5 minutes.
Add the rehydrated machaca to the pan and toss with the sautéed vegetables. If the mixture seems dry, add a few tablespoons of the reserved soaking liquid or beef broth to moisten; continue cooking 6–8 minutes so flavors meld and the beef warms through.
Season to taste with salt and black pepper, shredding the meat further with a spatula if needed to reach your preferred texture.
Warm the flour tortillas in a dry skillet or wrapped in a towel in the oven until pliable. Spoon the machaca filling down the center of each tortilla, fold and roll into burritos, and serve immediately.
