Preheat oven to 200°C. Combine onion, carrot and celery in a large flameproof roasting pan. Add ½ cup (125ml) water to pan.
Remove and discard neck from turkey. Pat dry inside and out with paper towel.
Use a vegetable peeler to remove 6 thick strips of rind from lemon. Pick the leaves from one-quarter of the thyme, sage and rosemary sprigs and reserve. Fill turkey cavity with lemon rind, bay leaves, garlic cloves and remaining thyme, sage and rosemary sprigs.
Tie the turkey legs together with kitchen string. Tuck wings under. Place over the vegetables in the roasting pan.
Combine the butter, crushed garlic, paprika, honey and oil in a small jug. Brush turkey all over with butter mixture. Season well. Cover pan tightly with lightly greased foil. Roast for 30 mins.
Remove foil. Reduce oven temperature to 180°C. Roast, basting every 30 mins, adding a little more water to the roasting pan if the liquid evaporates during cooking and covering with foil if necessary to prevent overbrowning, for a further 2½ hours or until turkey is golden and juices run clear when thigh is pierced with a skewer. Sprinkle turkey with reserved thyme, sage and rosemary leaves in the final 30 mins of cooking. Transfer the turkey to a platter and cover loosely with foil. Set aside for 15 mins to rest.
Meanwhile, heat the vegetable mixture in the roasting pan over medium-high heat, stirring, for 3 mins. Add the flour and cook, stirring, for 2 mins. Gradually add the stock, stirring until well combined. Bring to the boil. Reduce heat to low and simmer for 3 mins. Season. Strain the gravy through a fine sieve into a jug and discard the solids.
Serve the turkey with gravy and steamed or roast vegetables.