Place the 4 tablespoons of butter, garlic powder, onion powder, smoked paprika, thyme, salt and pepper in a bowl. Stir until combined.
Loosen the skin of the turkey. Spread half of the butter mixture under the skin of the turkey and spread half all over the top of the turkey.
Coat a large oval slow cooker with cooking spray and line the bottom of the slow cooker with the onion slices.
Place the turkey breast on top of the onion slices. Cover and cook on low for 6 hours or until a thermometer inserted into the thickest part of the breast registers 165 degrees F.
Transfer the turkey breast to a baking sheet. Preheat the broiler.
Broil the turkey for 4-5 minutes or until skin is browned and crispy.
Pour the liquid from the slow cooker through a strainer; discard the solids.
Melt 4 tablespoons of butter in a medium pan over medium heat. Add the flour and whisk to combine.
Cook the flour and butter mixture for 1 minute.
Pour 1 cup of strained drippings and 1 cup of chicken broth in the pan.
Cook for 4-5 minutes or until just thickened, stirring constantly. Season to taste with salt and pepper.
Slice the turkey and serve with the gravy. Garnish with fresh herbs if desired.