Pat the turkey breast completely dry with paper towels to ensure the butter rub adheres.
In a small bowl, mix the softened butter, minced garlic, thyme, rosemary, sage, salt, pepper, and paprika until a smooth paste forms.
Scatter the chopped onion and garlic across the bottom of the slow cooker insert and pour in the chicken broth.
Gently loosen the skin of the turkey breast with your fingers and rub half of the butter mixture underneath directly onto the meat.
Rub the remaining butter mixture over the outside of the skin.
Place the turkey breast on top of the onion bed in the slow cooker.
Cover and cook on LOW for 5 to 7 hours, or until the internal temperature reaches 165°F in the thickest part of the meat.
Remove the turkey from the slow cooker and let it rest on a carving board for at least 20 minutes before slicing.
Optional: For crispy skin, transfer the cooked turkey to a baking sheet and broil for 3-5 minutes until golden brown.