Step 1 Wipe the turkey inside and out with a damp paper towel. Place the turkey on a tray and rub the skin thoroughly with the cut lemon.
Step 2 In a small bowl, mix together half of the olive oil along with the garlic, chili pepper, salt, black pepper and nutmeg. Rub the marinade evenly inside and outside the bird, reserving any that remains. Put the bunch of rosemary in the cavity and truss the bird. Cover it loosely with foil and leave for 1 hour at room temperature.
Step 3Heat the oven to 375 degrees.
Step 4In a large roasting pan, toss the onions with the remaining olive oil and set the turkey on the onions, breast-side-down. Baste the turkey with remaining marinade. Roast it, basting often, for about 1 hour until the back (upper side) of the bird is well-browned. With a large spoon and fork (and maybe the help of another person), turn the turkey breast-side-up. Baste the turkey from time to time and roast until meat starts to shrink from the drumstick bones and the juices run clear, 45 minutes to an hour. A meat thermometer inserted in the thigh should read 160 degrees. If the turkey colors too quickly, tent it loosely with foil. If the onions start to scorch toward the end of roasting, add a cup of chicken broth to the pan. While the turkey roasts, prepare the mushrooms.