In a small bowl, mix together the softened butter, garlic powder, onion powder, poultry seasoning and salt and pepper until well combined.

Loosen the skin of the turkey (do not remove it). Spread half of teh butter mixture under the skin. Spread the remaining butter mixture over the outside of the turkey breast.

Spray a large slow cooker with non stick cooking spray. Lay the onions in the bottom of the slow cooker. Place the turkey breast on top of the onion slices.

Cover and cook on low for 6-7 hours, or until a meat thermometer inserted in the thickest part of the turkey reads 165 degrees F.

Transfer the turkey breast to a large baking sheet and preheat the broiler. Broil for 4-5 minutes, or until skin is browned and crispy. Cover and let stand for 10 minutes.

Meanwhile, pour the liquid from the slow cooker through a fine mesh strainer. Discard the solids.

In a medium saucepan, melt 4 tablespoons of butter over medium heat, stir in 4 tablespoons flour. Whisk in the strained drippings, plus 1 cup of chicken or turkey broth. Cook for 4-5 minutes, until gravy is thickened. Season with salt and pepper, to taste.

Garnish turkey with fresh herbs, if desired, before slicing and serving.