• Remove 1 3-4 pound boneless turkey breast from its packaging. If it is wrapped in netting, check the package directions to determine whether the netting should remain on during cooking. Pat the turkey dry with paper towels.
• In a small bowl, combine 1 teaspoon kosher salt, ½ teaspoon ground black pepper, 1 teaspoon garlic powder, 1 teaspoon onion powder, and 1 teaspoon poultry seasoning.
• Rub the seasoning mixture all over the turkey breast.
• Arrange the sliced onion and apple wedges in the bottom of a 6-quart slow cooker.
• Place the seasoned turkey breast on top of the apples and onions.
• In a medium bowl, whisk together 1½ cups apple cider, 2 Tablespoons Dijon mustard, 2 Tablespoons light brown sugar, and minced 3 cloves garlic.
• Pour the apple cider mixture around the turkey breast.
• Spread 4 Tablespoons unsalted butter evenly over the top of the turkey.
• Cover and cook on LOW for 5–6 hours or HIGH for 3–4 hours, or until the thickest part of the turkey reaches 165°F. Begin checking toward the earlier end of the cooking range to avoid overcooking.
• Carefully transfer the turkey breast to a cutting board or baking sheet and let it rest for 10–15 minutes.

Optional Broiled Finish

• For a beautifully browned exterior, transfer the cooked turkey breast to a foil-lined baking sheet.
• Place under the broiler for 3–5 minutes, watching it closely, until the outside is golden brown.
• Remove from the oven and allow the turkey to rest while you prepare the gravy.

Apple Cider Gravy

• Strain the liquid from the slow cooker through a fine-mesh strainer into a measuring cup. Reserve 2 cups of the cooking liquid. (You may want to save a little more just in case.)
• Melt 3 Tablespoons unsalted butter in a medium saucepan over medium heat.
• Sprinkle in 3 Tablespoons all-purpose flour and whisk continuously for about 1 minute.
• Slowly pour in the 2 cups reserved slow cooker cooking liquid, whisking constantly to keep the gravy smooth.
• Bring the mixture to a gentle simmer and cook for 3–5 minutes, whisking frequently, until the gravy has thickened.
• Season with ½ teaspoon black pepper and taste before adding additional salt.

To Serve

• Slice the turkey breast against the grain.
• Arrange the turkey slices on a serving platter and spoon some of the warm apple cider gravy over the top.
• Garnish with parsley and additional apple slices, if desired. Serve the remaining gravy on the side.