Wash the beef well. You can use a slow cooker or an oven to boil the beef. However, in a slow cooker, the beef becomes moist and soft. Pour oil in the cooker and warm it. While it is warming, season one side of the beef with salt and pepper. Place the beef in the oil with the seasoned side down. While that side is being seared, season the other one. After two minutes, flip over the beef to sear the other side as well.
Now add the garlic, all spices and the tomato paste to the cooker over the beef. Fill the cooker with water to a level where it just covers the beef. Let the beef boil for three hours.
After three hours, check the beef for tenderness. If it is, remove and place on a cutting board. Use two forks to shred it.
Fill tortilla with the shredded beef and add toppings like avocado, lime, cheese and cilantro. You can also thicken the cooking liquid in the cooker and drizzle a small amount over the shredded beef to make it richer.