In a small bowl, soften the butter in the microwave for a few seconds if it's not already soft (don't completely melt it). Spread a little bit of the butter in the bottom of your slow cooker, then place the sliced onions in the slow cooker.
Remove the turkey breast from the packaging and discard any twine used for trussing, gravy inserts, etc. Pat the turkey breast dry with paper towels. I like to place paper towels on a rimmed baking sheet, place the turkey breast on top, and pat the top dry.
To make the rub, add the kosher salt (2 teaspoons), black pepper (1/2 teaspoon), garlic powder (1 teaspoon), dried oregano (1 teaspoon), dried thyme (1 teaspoon), paprika (1 teaspoon), and turmeric, if using (1/2 teaspoon) to the softened butter and stir together to form a paste.
Spread a little of the rub on the bottom of the turkey, then place it in the slow cooker. If the turkey comes with skin, lift it up gently and spread some of the rub under the skin. Place the skin back on top and spread the majority of the rub all over the top of the turkey breast evenly.
Cover and cook on low for 4 hours.
Optional: If you want crispy skin, place the turkey skin-side up on a rimmed baking sheet and broil for 5-10 minutes, or until the skin is crisped to your liking.
Allow to rest for 20 minutes before slicing.
If using a butterball turkey breast, or other pre-brined turkey breast, you may want to use only 1 teaspoon of kosher salt instead of 2, since it's already salted.
To make gravy from the drippings: melt 2 tablespoons butter (or fat from the drippings) over medium-high heat. Add 2 tablespoons flour. Whisk together to form a roux, cook for at least one minute. Pour in the drippings (about 1 cup) and continue to whisk and cook until thickened, about 3-5 minutes. Supplement with chicken stock if you need more drippings for a double or triple batch. (Here's more on how to make gravy from turkey drippings).
The turmeric in this recipe gives the turkey a golden color and helps add flavor. If you don't like the yellow color, simply omit it.
Be sure to use a fully THAWED turkey breast. Often, turkey breasts come frozen. You will need to plan ahead to defrost it- keep it in the fridge for about 24 hours per 5 lbs. of meat to defrost before slow cooking. It's not safe to cook frozen meat in a slow cooker, as the inside will not fully cook quickly enough.