Flavorful rosemary-lemon oil is great for the turkey. You can also use it to season vegetables before roasting or after steaming, or to dress up a vinaigrette.
Let turkey sit at room temperature 30 minutes. Preheat oven to 400 degrees, with rack in lower third. In a small saucepan, heat oil, rosemary, garlic, and lemon zest. Bring to a simmer over medium-high and cook until garlic is soft and lemon is slightly shriveled, 1 minute. Remove rosemary-lemon oil from heat and let cool.
Tuck wing tips underneath body of turkey. Season inside of turkey with salt and pepper. Loosely fill large cavity and neck cavity with stuffing; fold neck skin over opening. Tie legs together with kitchen twine. Brush turkey all over with 1/2 cup rosemary-lemon oil and season with salt and pepper.
Place onion in a heavy-bottomed roasting pan. Set a roasting rack over onion and place turkey on top. Roast until turkey is golden brown, 45 minutes to 1 hour. Reduce heat to 375 degrees, add 1 cup water to pan, and roast, brushing turkey with remaining rosemary oil every 30 minutes, until an instant-read thermometer inserted in thickest part of a thigh reads 165 degrees, 1 1/2 to 2 hours (add some water, if pan becomes dry).
Transfer turkey to a platter or cutting board and loosely tent with foil; let rest 30 minutes. Reserve pan with drippings for
White Wine Gravy. To serve, scoop out stuffing and carve turkey.