Trim and discard the excess fat from the meat. This will ensure that you're not left with oily, greasy cooking liquid.
Coat the roast with olive oil, then salt and pepper the entire roast.
Mix all the other ingredients together, except the beer and the beef stock, and make a paste. Smear the paste all over the roast and set it in a slow cooker turned on low.
Gently pour the beer and beef stock into the slow cooker without rinsing the paste off the roast (substitute more stock for beer if desired). Close the lid and let the roast cook for 7 to 8 hours.
When the meat is tender and pulls apart easily, remove it from the crock pot, place it on a cutting board and pull the roast with two forks.
Skim any fat off the cooking liquid. Place the pulled meat in a serving dish and ladle the desired amount of sauce over it. All the flavors from the sauce will be absorbed by the meat. Serve warm with your favorite toppings on corn or flour tortillas, corn tostadas, corn taco shells or by itself with some rice and beans.