In a container or pot large enough to hold turkey, whisk sugar, salt and 8L water until sugar and salt have dissolved. Submerge turkey in brine, adding a heavy plate to keep turkey below brine. Refrigerate for 18-24 hours.
Preheat oven to 200°C (180°C fan-forced). Remove turkey from brine and thoroughly pat turkey dry. Stuff turkey with half the chopped onions, carrots and celery and tie turkey legs together with kitchen string. Place remaining onions, carrots, and celery in large roasting pan. Place turkey on a rack in the pan.
In a small saucepan, melt 100g of the butter with sage stems and Worcestershire sauce. Remove pan from heat. Brush some melted butter mixture all over turkey.
Roast turkey for 30 mins, or until skin is light golden. Reduce oven temperature to 180°C (160°C fan-forced) and roast turkey, basting every 30 mins with melted butter mixture, for 11/4-11/2 hours or until an instant-read thermometer reads 66°C when inserted into thickest part of breast.
Transfer turkey to a carving board (don’t clean out roasting pan) and rest at room temperature for 20 mins before carving.
Carefully pour pan drippings from roasting pan into a small measuring cup and set aside for 5 mins to allow fat to rise above juices. Spoon off fat and reserve pan juices. Heat the roasting pan on stove over medium-high heat. Add pan juices and stock and bring to a simmer, stirring to scrape up brown bits. Strain pan juices and discard solids.
In a medium heavy saucepan over medium-high heat, add remaining 50g butter and swirl to melt. Continue to swirl in pan for 2 mins or until butter is light golden brown. Stir in sage leaves and cook for 1 min or until they are crisp. Remove pan from heat and, using a slotted spoon, remove sage leaves from brown butter. Sprinkle sage leaves with salt and reserve.
Return pan to medium heat, add flour and cook, stirring, for 1 min. Whisk in pan juices and bring mixture to a simmer. Simmer for 3 mins or until gravy thickens slightly. Season gravy with salt and pepper.
Carve turkey and serve with gravy, crispy sage leaves and stuffing.