To make the turkey:

Preheat oven to 375 degrees F with the rack in the lower portion of the oven. Clean the turkey with cold water and then pat dry with a paper towel. Stuff the turkey cavity with the onion, orange, garlic and herbs. Tie the legs up using kitchen twine. Place the turkey on a roasting rack in a roasting pan and rub the turkey all over with the olive oil. Season the turkey liberally with salt and pepper. Place the turkey in the oven and roast for 2 hours, basting the turkey every 30 minutes with turkey stock. After two hours, or when the turkey reaches an internal temperature of 150 degrees when an instant read thermometer is inserted into the thigh, begin brushing the turkey with the elderberry syrup. Continue to glaze the turkey every 15 minutes or until the turkey reaches 165 degrees F and the elderberry syrup has created a glossy glaze over top. Remove from oven and let the turkey rest 20-30 minutes prior to carving and serving.

To make the syrup: 

Add all the ingredients to a large pot over medium-high heat and bring to a boil, then reduce to a simmer and cook for about an hour. Remove from heat and carefully strain through a fine mesh strainer or cheesecloth positioned high enough over a bowl to allow it to drain through. Once it has drained, place the liquid in a small pot over medium-high heat and simmer until the mixture reduces by about half and becomes the consistency of syrup, about 30 minutes. Let it cool completely before storing in an airtight container in the refrigerator.