Position an oven rack in the lowest position (remove the other racks); preheat to 350 degrees F. Remove the neck and giblets from the turkey and set aside for the gravy. Pat the turkey dry with paper towels and rub inside and out with 1 tablespoon each salt and pepper. Stuff the cavity with the onion, smashed garlic and rosemary sprigs. Tie the legs together with kitchen twine. Put the turkey on a rack set in a large roasting pan and tuck the wings under the body. Let sit at room temperature while you make the porcini butter.
Soak the porcini in 1 cup hot water until softened, about 10 minutes; drain. Put half of the porcini in a food processor (finely chop the rest to use in the gravy). Add the whole garlic cloves and chopped rosemary to the food processor; pulse to make a paste. Add the butter, Worcestershire sauce, 2 teaspoons salt and a generous amount of pepper; pulse until smooth. Rub the turkey inside and out with the butter. Transfer to the oven and roast 1 hour. Meanwhile, start your gravy, adding the chopped porcini in along with the stock.
After the turkey has roasted 1 hour, baste with the drippings. Continue roasting, basting every 30 minutes, until the skin is golden brown and a thermometer inserted into the thigh registers 165 degrees F, 1 1/2 to 2 more hours.
Transfer the turkey to a cutting board and let rest 30 minutes before carving; use the drippings for the gravy.