Remove the turkey from its packaging, following the package directions for netting, and pat dry. Combine the salt, ½ teaspoon pepper, garlic powder, onion powder, and poultry seasoning, and rub over the turkey.
Arrange the onion and apple in a 6-quart slow cooker. Place the turkey on top.
Whisk together the cider, Dijon, brown sugar, and garlic, and pour around the turkey. Spread 4 tablespoon softened butter over the top.
Cover and cook on LOW 5–6 hours or HIGH 3–4 hours, until the thickest part reaches 165°F. Transfer to a cutting board and rest 10–15 minutes.
Optional: Broil on a foil-lined baking sheet 3–5 minutes, until golden brown.
Strain the cooking liquid through a fine-mesh strainer and reserve 2 cups.
Melt the remaining 3 tablespoon butter in a medium saucepan over medium heat. Whisk in the flour and cook 1 minute. Gradually whisk in the reserved liquid and simmer 3–5 minutes, until thickened. Season with the remaining ½ teaspoon pepper and salt to taste.
Slice the turkey against the grain and top with gravy. Garnish if desired, and serve the remaining gravy on the side.
Notes
Use sweet apple cider, not apple cider vinegar or apple juice.Begin checking the temperature at the early end of the cook time.If the turkey was brined or injected with a salt solution, taste the gravy before adding salt.Refrigerate the turkey and gravy separately in airtight containers for up to 4 days. Sliced turkey can be frozen with a little cooking liquid for up to 3 months.
All nutritional information provided is an estimate. For specific dietary needs or concerns, we recommend consulting a nutrition calculator or a qualified expert.