Preheat the oven to 180°C (160°C fan-forced).

To make bacon, onion and sage stuffing, heat butter in large frying pan; cook the bacon, stirring, until browned. Add the onion and celery, cook, stirring, until soft. Cool 10 minutes. Combine onion mixture, herbs, breadcrumbs, egg, salt and ground black pepper in a large bowl; mix well.

Rinse the turkey inside and out under cold water; pat dry with absorbent paper. Fill the large cavity with seasoning. Tie the legs together with string. Fill the neck cavity with stuffing and secure the skin over the opening with toothpicks.

Tuck the wing tips under the turkey.

Place the turkey on an oiled wire rack in a flameproof baking dish. Dip the muslin in the melted butter, place the muslin over the turkey, cover dish tightly with foil. Bake for 2 hours.

Remove the foil and muslin from the turkey, brush with the combined pan juices and olive oil. Bake the turkey, uncovered, for a further hour or until browned and cooked through. To test, insert a skewer sideways into the thickest part of the thigh. Remove the skewer and press the flesh to release the juices - if the juices are clear the turkey is cooked. Or, insert a thermometer into thickest part of turkey thigh, without touching the bone; it should read 90°C. Test the stuffing as well; it should read at least 75°C. Remove turkey from dish, sprinkle with the salt and thyme and stand, covered with foil, for 20 minutes.

Meanwhile, drain the pan juices from the dish into a large jug, skim the fat from the top of the juices, reserve juices, discard fat.

To make port gravy, place the same baking dish over a medium heat, melt the butter in the dish and cook the onion, celery and garlic, stirring, until soft. Stir in the flour and cook, stirring, until the flour is well browned. Remove from the heat, stir in the port; return to the heat. Gradually stir in the combined reserved pan juices and stock. Cook, stirring, until mixture boils and thickens. Strain gravy into a large jug.

Serve turkey with port gravy, if desired.