Cut each onion Into 4 large chunks and place in the bottom of the slow cooker.
Add the rest of the ingredients (besides the turkey) to a bowl and mix together to form a paste.
Place the turkey in the slow cooker and pat dry with paper towels.
Take the butter mixture and rub it all over the turkey.
Cover the crock pot and set it on low for 6 hours. After the 6 hours, check the internal temperature. If It's at least 165°, the turkey is done.
To crisp the skin, place the turkey breast on a baking sheet and broil in the oven for about 5 minutes. Let rest for 15 minutes then carve.