I suggest seasoning your goose the night before you need it. You can use my old-English Christmas rub or simply some good sea salt and pepper. Rub it all over the goose, both inside and out, and leave in the fridge for a few hours, preferably overnight, to let all that flavour get into the bird. This will also help to give you lovely crispy skin later on.
Before roasting, make sure the shelves in your oven are on the middle and bottom levels. Preheat the oven to 180°C/350°F/gas 4. Stuff the goose with the rosemary, the clementines and the cinnamon stick, then wrap the legs in tinfoil to stop them from burning. Using a fork or the tip of a sharp knife, prick the fatty lump above the goose's cavity to help the fat melt out.
Next, place your goose, breast-side up, on to the bars of the middle shelf in the oven, and place a wide roasting tray (about 5cm deep) on the shelf below the bird to catch the fat as it drips. Roast for about 2 hours, then add the carrots, onions and celery to the tray; this will become the base for a really tasty gravy. Cook for another hour, then take it out of the oven and transfer it to a large board. Cover loosely with foil and leave to rest while you make your gravy. Remove the tray of veg and fat from the oven and sieve it, keeping back the lovely roasted bits of veg for the gravy.
Pop the veg back into the tray they cooked in and place on the hob over a medium heat. Sprinkle over a couple of spoonfuls of flour and give everything a good stir, scraping any sticky goodness from the bottom of the tray. Cook it out for a minute or two, then mash up all the veg and add the port or red wine and about 500ml of the stock. Let everything simmer until you have a consistency you like, then sieve the gravy, pushing all the goodness of the veg through the sieve. You can always loosen it with a little extra stock later if you wish. Let it cool, then keep it in the fridge until needed.
By now, the meat should be cool enough to handle, so start stripping the carcass. It's easiest to slice the breast off with a knife then pull the remaining meat off using a pair of forks the way they do in Chinese restaurants. Transfer the meat to a deep earthenware dish or bowl as you go, then cover the meat with the strained goose fat; this ensures no air can get in. Scatter some sage leaves over the top for added flavour and press them down so they are just covered in the fat. Place in the fridge until Christmas Day.
If you have any leftover fat, save it for your roasties. Pour the sieved goose fat into an airtight jar and add a few rosemary and sage sprigs to it. It'll keep in the fridge for a couple of weeks and will add amazing flavour to roast veg.
About half an hour before serving, pop the goose meat into a low oven for about 25 minutes. Once it's all hot, carefully drain away any excess fat and reheat your gravy. Fry the sage leaves in a splash of olive oil, until nice and crispy, then scatter over the goose with the pomegranate seeds.