In a mixing bowl, whisk together the maple syrup, soy sauce, apple cider vinegar, Dijon mustard, garlic powder, onion powder, black pepper, salt, cinnamon, and nutmeg until well combined.
Place the turkey breast in the slow cooker and pour the maple syrup mixture over it, ensuring the turkey is well coated.
Cover the slow cooker and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the turkey is cooked through and tender.
Once cooked, remove the turkey from the slow cooker and let it rest for about 10 minutes before slicing.
Serve the turkey slices drizzled with the cooking liquid from the slow cooker and garnish with fresh parsley if desired.