Thoroughly mix the salt, cumin, black pepper, oregano, and chili powder in a bowl. Set aside.
Lay out two large sheets of tin foil in a large pan or bowl (the bowl helps keep the liquids from spilling out while you wrap the beef). Use enough foil to completely wrap and seal the meat (I make an '+' pattern out of two large sheets).
Rub the seasoning mixture into the meat, along with the minced garlic. Add the beef broth in the center of the tin foil (this is where placing the foil in a large bowl or pan helps). Place meat on top of the broth, surround with the onions and add the two bay leaves on top. Wrap tightly and place in the refrigerator overnight. Make sure that your foil pack is water-tight. You don't want any liquids to escape. It helps to roll the top first, then flatten out the sides and roll them up.
The next morning, place the wrapped beef cheeks in your slow cooker and pour water in to about 1 inch from the top of the crock pot. Cover, turn on low, and let cook for at least 10 hours.
Carefully remove the foil packet from the slow cooker. Unwrap, being careful not to spill any hot liquid on your self (I like to put it into a large bowl and tear open the foil with two forks). You can shred the meat with two forks, but I prefer to finely chop it with a sharp knife. Be sure to pick out any tough bits of cartilage or membrane that you find.
When planning your meal, you should expect that beef cheek meat will reduce in size by a little less than one-half during the cooking process.