Make the stuffing: Heat oil in a large frying pan, add leek; sauté for 2 minutes. Add coriander, cranberries, orange zest and juice; simmer for 1 minute. Mix together with remaining stuffing ingredients and season well.
Preheat the oven to 180°C/Fan 170°C/350°F/Gas Mark 4. Place turkey on a board; use your fingers to gently separate the skin from the flesh all over the breast. Beat butter, orange zest and 2 tbsp/30ml of orange juice together with seasoning; push two thirds of the orange butter under the breast skin. Spoon stuffing into neck cavity; tuck skin flap under turkey and secure with cocktail sticks.
Weigh turkey, then calculate cooking time: allow 20 minutes per kg + 70 minutes. Place upside down in a roasting tin; smear over remaining butter. Loosely cover with foil.
Roast the turkey for calculated time. At half time, turn over turkey, baste with pan juices. About 45 minutes before end of roasting time, remove foil to allow skin to brown and add onion and orange wedges to roasting tin. Stretch bacon with back of a knife and layer criss-crossed turkey with bacon; return to oven. When cooked, remove turkey from oven; transfer to a warm serving plate with the onion and orange wedges, cover with foil and leave to rest for 15-20 minutes – this makes carving easier.
Make the gravy: spoon off excess fat from the juices in the roasting tin. Place tin on a low heat; add flour and cook, stirring for 1 minute. Gradually stir in stock and cranberry juice. Bring to the boil stirring; simmer for 5 minutes, strain through a sieve. Pour into a warm gravy boat. Serve the turkey crown with roast potatoes and seasonal vegetables.