Remove the turkey from its packaging, leaving the netting intact, and pat it dry with paper towels. Let it sit at room temperature for 30 minutes.

Place the quartered onion and lemon at the bottom of your slow cooker.

Mix the softened butter with minced garlic, thyme, parsley, salt, pepper, and sage until well-combined.

Generously rub the herbed butter mixture all over the turkey breast and place the turkey on top of the onions and lemons in the slow cooker.

Cover the slow cooker and cook on LOW for 5 hours or until the turkey reaches an internal temperature of 165°F.

Transfer the turkey to a platter and let it rest for 10 minutes before slicing.

Strain the juices from the slow cooker into a saucepan. Add the reserved gravy packet. In a separate bowl, whisk together the broth and cornstarch until smooth, then stir into the saucepan with pepper, onion powder, sage, parsley, and Dijon mustard. Cook until the gravy thickens.

Pour the gravy over the sliced turkey before serving. If serving later, return the sliced turkey to the crock pot with the gravy to keep it warm.