Butter the crockpot or line with a crockpot liner. Place the onion wedges, garlic, thyme and rosemary sprigs on the bottom of the crock.
Combine all the ingredients for the rub and stir until well combined.
Pat the turkey breast dry. If there is a net around it, you can leave it on or remove it.
Press the rub onto the turkey breast. The butter will harden, which is fine. Press around the top of the turkey breast.
Place the turkey on the onions and place the lid on the crockpot.
Set on Low for the time needed to cook however many pounds your turkey breast weighs (see Notes below for times). Don't cook on High.
A meat thermometer should read 165°F. Remove the breast from the pan and place in a 2-quart oven safe pan.
Set the broiler for 500°F and place the turkey under the broiler for 5 minutes. Watch carefully. You want the skin to have a nice crisp but not burn. If your turkey breast is skinless, placing it under the broiler will brown the outside and seal in the moisture.
Remove the turkey and allow it to rest for 10 minutes before slicing.