Preheat the oven to 190ºC/375ºF/gas 5. Find the largest roasting tin that will fit in your oven.
Scrub the beetroots, halving or quartering any larger ones. Peel and smash the garlic, then place in a bowl, pick in the thyme leaves and combine with the balsamic and a lug of olive oil. Season well with sea salt and black pepper, then tip into one corner of the roasting tray.
Cut the squash into wedges, then deseed. Crush the coriander seeds and chilli, then peel and smash the garlic. Place in the same bowl, then strip in the rosemary and toss together with the cinnamon stick and a lug of olive oil, and season. Tip into another part of the roasting tray.
Peel the carrots and parsnips, then pick over the thyme leaves. Toss with the zest and juice of the clementines and a splash of olive oil. Tip into another part of the roasting tray.
Peel and cut the celeriac into wedges. Crush the fennel seeds and tear the sage, then toss with the celeriac and a little olive oil and add to the roasting tray, again keeping it separate.
Peel and halve the red onions, toss with the thyme leaves, balsamic and a little olive oil, then add to the roasting tray.
Wash the turnips, tear the bay leaves, then toss with a lug of olive oil and a nice grating of nutmeg. Add to the roasting tray, ensuring each veg group is not too clumped together in the tray.
Cover the tray with tin foil and roast in the oven for 50 minutes, or until the vegetables are tender and golden, removing the foil for the last 20 minutes.
For the horseradish crème fraîche, finely grate the horseradish into a bowl, then mix with the crème fraîche, 1 tablespoon of extra virgin olive oil and a squeeze of lemon juice. Season to taste and set aside.
To make the wild mushroom sauce, place a large frying pan over a high heat and pour in a couple of lugs of olive oil. Tip in the mushrooms, tearing any larger ones, and give it a shake to coat them in the oil. Cook for 5 minutes, or until beginning to soften.
Peel, finely slice and add the garlic, and strip in the thyme leaves. Give the pan a good shake and cook for 4 minutes, or until the mushrooms are coloured, then add the butter and a squeeze of lemon juice.
Reduce the heat to medium-low and stir in the cream, adding a splash of water to loosen to you desired consistency, if needed.
To make the polenta, pour 2 litres of water in a large pan and bring it to the boil over a medium heat. Slowly pour in the polenta in a steady stream, whisking until incorporated. Continue stirring over the heat for 10 minutes, or until it easily comes away from the edge of the pan.
Stir in the butter and grate in the Parmesan, beating until creamy and adding a splash of water to get it to the right consistency, if needed.
Season the polenta to taste, then immediately pour it onto a large board or platter and scatter over a few chunks of Stilton. Toss the roasted veg in their cooking juices, then spoon them over the polenta.
Scatter over the celery leaves, a grating of truffle or a drizzle of truffle oil (if using) and spoon over any remaining juices from the tray. Delicious with mostarda di Cremona and salsa verde.