Preheat the oven to 200°C/450°F/Gas mark 4.
Take the giblets out of the turkey and wash the turkey thoroughly, inside and out.
Pat the outside of the turkey dry using paper towels.
Cut the onion and lemon into wedges.
Stuff the cavity with the lemon, onion, garlic cloves, thyme, rosemary and sage.
Tie the legs together using kitchen twine or use its own skin to tuck the legs together, and tuck the wings snuggly under the body.
Mix the olive oil with the melted butter, add the Erythritol and a pinch of salt and pepper. Whisk well and rub the outside of the turkey with the mixture.
Fit a roasting rack inside a baking pan and place the turkey on the rack, breast side up.
Cook in the preheated oven for 30 minutes, then reduce the oven temperature to 350°F.
Keep roasting basting with the juices from the pan, until the juices run clear when you cut between the turkey leg and the thigh, for approximately 3 hours.
Remove from the oven and cover with tin foil, allowing the turkey to rest for 20 minutes before carving and serving.