Coat beef cheek meat with olive oil. Rub salt, cumin, and pepper into meat. Wrap meat in aluminum foil and refrigerate, 4 hours to overnight.
Pour water into a slow cooker.
Arrange onion and garlic around beef cheek meat in the aluminum foil. Wrap foil tightly around meat and vegetables. Add a second sheet of aluminum foil around meat mixture, sealing tightly; place in the slow cooker.
Cook on Low, adding more water if it has evaporated, until meat is very tender, 7 to 8 hours. Remove foil packet from slow cooker and shred meat using 2 forks.