To make the stuffing, heat 1 Tbsp olive oil in a frying pan. Add the onion and garlic and cook over a low heat for about 5 minutes or until the onion is translucent. Set aside to cool.
Blanch the spinach in boiling salted water until just wilted, then drain and rinse under cold water. Squeeze the excess water from the spinach using a paper towel and chop finely.
In a large bowl, combine the onion mixture with the spinach, ricotta, herbs, celery, raisins and butter. Stir in the eggs, season and mix well with a fork.
Spoon stuffing into the neck cavity just prior to cooking. Make sure to stuff evenly to prevent uneven cooking. Season and coat the exterior of the turkey with olive oil, salt and pepper.
Place the turkey in a large baking tray and roast for 30 minutes. Reduce the oven temperature to 150°C and cook the turkey for 4 hours or until cooked.
Using a meat thermometer. ensure that the internal temperature of the breast is 74°C and the legs 80°C. If you do not have a meat thermometer, insert a clean skewer into the thickest part of the thigh. If the juices run clear the turkey is cooked, if they run pinkish, return to the oven and cook for a little longer.
Allow to rest, which makes the meat easier to carve. Set turkey in a warm place, lightly covered with foil, for at least 20-30 minutes before carving and serving.