Spray the inside of the Crock Pot with nonstick cooking spray.
Slice the onions in half and place them face down on the bottom of the slow cooker. They will help hold the turkey up out of the liquid.
Add a few sprigs of thyme on the bottom.
Add the butter, garlic, onion, salt, pepper, and rosemary to a small or medium-sized bowl and mix well.
Place the turkey in the slow cooker and dry off with some paper towels to help the butter stick.
Spread the butter over the top of the turkey.
Pour the chicken broth into the side of the Crock Pot, being careful not to pour it on the turkey.
Place the lid on top and cook for 7 hours on the low setting.
Once the turkey is done cooking, check the temperature with a meat thermometer. The temp should be at least 165 degrees in the thickest part of the turkey breast.
Place the turkey under the oven broiler for 5 to 10 minutes (in the crock only) until the top is crispy. (Watch it close; it can burn fast, and make sure you remove the Crock Pot lid).