Preheat the oven to 390ºF. Slice a lid off the top of the pumpkin, and remove the seeds and fibrous flesh from the inside, keeping the top to put back on later.
In a large saucepan (with lid), fry the onion gently in the oil until softened, then add the 2 minced garlic cloves, the cranberries, spices and clementine zest. Stir in the rice, turning till it becomes glossy in the pan.
Pour in the stock and let the pan come to the boil, then clamp on the lid and turn the heat down to the lowest it will possibly go. Cook for 15 minutes.
Cut the remaining clove of garlic in half and rub the inside of the pumpkin with the cut side of each half, then, using your fingers, smear some salt over the flesh inside as well.
The rice stuffing will be quite damp and not very fluffy at this stage, but check it for seasoning - adding more spice, salt or pepper if wanted - and then spoon it into the garlic - and salt-rubbed pumpkin cavity and tamp down well. Press the pumpkin lid back on top and squeeze it down as firmly as you can (it will sit up a bit proud of the top).
Stand the pumpkin on a double layer of foil, wrapping the foil 2 inches up around the sides and scrunching it there, to keep the pumpkin out of direct contact with the water later.
Place the stuffed, partially wrapped pumpkin in a roasting tin and pour in freshly boiled water to a depth of 1 inch. Cook the pumpkin for about 2 hours by which time it should be tender when pierced.
Meanwhile, prepare the gingery tomato sauce.
Put the onion, garlic, dried ginger and fresh ginger into a processor and blitz to a pulp.
Heat the butter and oil in a deep, wide pan, then add the onion-garlic mixture. Cook over a low heat for about 10 minutes, stirring occasionally so that it doesn't burn.
Add the passata and water to the pan, and season with the sugar, salt and pepper. Cook for 15 minutes, at a gentle simmer, then taste for seasoning before decanting into a warmed jug or gravy boat and taking to the table, for people to pour over their slices of stuffed pumpkin.
Take the pumpkin out of the roasting tin and let it sit for about 10 minutes before you slice it into segments like a cake.