Rinse the turkey and pat it dry with paper towels.

❗YES! The turkey breast is frozen! That is why you are cooking it for so long. DO NOT BE IN A HURRY! 
Rub the turkey all over with the oil.
Sprinkle the breast lightly with salt and pepper.
For more highly seasoned meat, sprinkle 1 tablespoon dried herbs de Provence or dried Italian seasoning over the breast along with the salt and pepper. 
Place the breast, meaty side up, into a 5-quart slow cooker.
Peel the onion, cut it into quarters, and place the pieces around the edges of the pot.
Add the liquid if you are using it.
Peel the garlic cloves and place them around the sides of the cooker.
Cover cooker and cook at low for 9 hours, or until a meat thermometer inserted in the center of the meat registers 165°. 🎯
Remove the breast from the cooker and let stand at least 10 minutes before slicing. (The internal temperature will continue to rise while the bird is standing.)
Discard the onions and garlic.
Serve half the breast sliced, preferably without the skin.

Reserve the remaining breast half for use in other recipes:
Discard the turkey skin.
Remove the breast meat from the bone and coarsely chop.
You should have about 3 1/2 cups.
Place the meat in a freezer-weight plastic bag, and freeze for up to 2 weeks.