Heat the oven to 190C/170C fan/gas 5. Bring a large pan of salted water to the boil.
For tray one, mash the butter, 1 tsp mace, half the sage and all the garlic with some seasoning. Use your hands to separate the turkey meat and skin to create a pocket. Spread half the spiced butter under the skin and smooth into an even layer, taking care not to pierce the skin. Rub the remaining butter over the skin and season again. Put the onion slices in your second largest roasting tin (tray one), and sit the turkey on top. Roast for 1 hr-1 hr 30 mins in the middle of the oven (1 hr for a 2kg crown, 1 hr 30 mins for a 3kg turkey crown), basting with the butter that pools in the bottom halfway through.
Meanwhile, for tray two, cook the potatoes in the boiling water for 8 mins, then add the parsnips and cook for another 3-5 mins until just tender. Lift both out of the pan using a slotted spoon into a wide bowl and leave them to steam-dry. Put the carrots in the pan and cook for 5 mins. Add the sprouts and cook for 2-4 mins more until just tender. Reserve a jug of the cooking water (about 500ml) and drain the carrots and sprouts. Leave to steam-dry.
To make the stuffing, mix the sausagemeat, cranberry sauce, prunes, and remaining mace and sage with seasoning. Mix well with your hands, then roll into 16 stuffing balls. Wrap each with a halved bacon slice and chill until needed.
When the turkey has had its time, baste again and add the stuffing balls around it, as far apart as possible to ensure they brown. Move to the bottom of the oven, roasting for another 30 mins. Put a shelf above for the potatoes, and heat the oil in your largest shallow roasting tin (tray two) at the top of the oven for 10 mins.
Season the potatoes and parsnips, and sprinkle over 2 tbsp flour. Put a plate over the bowl, hold it down, and shake to toss the spuds and parsnips in the flour. Gently lower them into the hot oil, and spoon over the fat. Roast for 30 mins, turning them halfway through.
When the turkey has had 1 hr 30 mins (or 2 hrs for a 3kg turkey crown), check it’s cooked through – a digital cooking thermometer should read 70C when inserted into the middle of the breast, and there should be no pink juices when pierced with a skewer. Remove from the tin and leave to rest along with the stuffing, covered with foil (you can warm them up in the second tray if you need to later on).
Turn the oven up to 220C/200C fan/gas 7 and roast the potatoes for 10 more mins until lightly golden. Toss the carrots, sprouts and bay leaves into the roast potato tray, and roast for another 30 mins while the turkey rests, until all the veg is golden and tender. Add the stuffing balls for the last 5 mins to crisp and warm through again, if you like.
Add the port or wine to the turkey tray with the roasted onions, and put over a low heat on the hob (or transfer to a pan if your tray isn’t flameproof), scraping any bits off the bottom. Add most of the reserved cooking water and whisk everything together. Mash the 1 tbsp butter with the 2 tbsp flour to make a paste, then whisk into the gravy. Simmer for 10 mins until thickened. Season, and strain into a gravy jug.