In a small bowl, mix the salt, garlic powder, smoked paprika and pepper until well combined. Put the onions in the bottom of a 6-quart slow cooker with the garlic cloves.
Slowly work your fingers under the skin of the turkey breast to separate the skin from the breast, being careful not to tear the skin. Rub the butter under the skin, covering the entire breast as evenly as possible, then rub the outside of the skin with the garlic powder mixture.
Place the turkey breast on top of the onions and garlic in the slow cooker and set the slow cooker to low. Cook until the meat is falling apart, 6 to 8 hours depending on the size of the breast.
Remove the turkey and any bones from the slow cooker. Pull the meat off of the bone and place the turkey back into the juices left in the slow cooker. Using 2 forks, shred the meat in the juices and allow to rest for 5 to 10 minutes before serving.