Thoroughly pat the turkey dry. Using your fingers, loosen skin over breasts and around thighs of turkey. Sprinkle salt between loosened skin and meat. Sprinkle the outside of the turkey with salt. Stuff turkey cavity with half the sage and half the thyme. Place turkey on a roasting rack and place in the fridge, uncovered, to air-dry for at least 12 hours.
Transfer the turkey and rack to a flameproof roasting pan. Set aside for 1 hour to reach room temperature. Preheat oven to 220°C (200°C fan-forced). Brush the turkey with oil. Pour enough water into the pan to just cover the base.
Roast the turkey for 45 mins (checking occasionally to add more water to the bottom of the pan if it dries out). Rotate turkey 180 degrees. Roast for 45-55 mins or until the turkey is cooked through and an instant-read thermometer reads 74°C when inserted into the thickest part of the thigh.
Sprinkle remaining sage and thyme over the centre of a carving board. Arrange the turkey over the herbs (don’t clean out the roasting pan). Set aside for 30 mins to rest before carving.
Meanwhile, to make the mustard-peppercorn gravy, pour pan juices into a measuring cup. Set aside for 10 mins to allow fat to rise. Spoon off fat, reserving pan juices. Set the roasting pan on the stove over medium heat. Add the pan juices and stock. Bring to a simmer, stirring to scrape any brown bits on the base of the pan. Strain the mixture through a sieve into a jug. Discard solids. Measure 3 cups (750ml) stock mixture, reserving any extra in a small jug.
In a medium heavy-based saucepan over medium heat, melt the butter. Add shallots and cook, stirring frequently, for 2 mins or until shallots soften. Stir in flour. Cook, stirring, for 2 mins. Whisk in stock mixture, mustard, peppercorns and Worcestershire sauce and bring to a simmer. Cook for 5 mins or until the gravy thickens slightly. Remove from heat and season with salt and pepper. Keep gravy warm, adding reserved stock mixture to thin out if necessary.
Carve the turkey and serve with gravy.