Scrub the celeriac clean, using a brush to clean away any soil from the root. Tear off a double layer of wide tin foil and place the celeriac in the middle, root side up.
Rub with olive oil, sea salt and black pepper, sprinkle over the thyme sprigs and 6 bay leaves, then bash 4 whole cloves of garlic and scatter over. Pull the sides of the foil up really tightly around the celeriac and scrunch around its shape, leaving it open at the top.
Place the butter on top of the celeriac so that it melts down and around it as it cooks, then fold the foil over really tightly to seal. Place in an ovenproof dish and roast for around 2 hours, or until tender.
Meanwhile, cook the pearl barley at the appropriate time according to packet instructions.
Peel and finely slice the onion and remaining garlic, place in a large frying pan on a low heat with a lug of olive oil, and fry for around 10 minutes, or until softened, stirring occasionally.
Finely slice the mushrooms and add (your pan will be very full, but trust me, they will cook down nicely). Cook for around 20 minutes, or until golden, continuing to stir occasionally.
Crumble in the stock cube, add the remaining bay leaf and pour in 200ml of boiling water. Simmer and reduce until the liquid has nearly gone, then stir in the cream and mustard and simmer for a further 5 minutes. Season to perfection and keep warm until needed, being careful not to let it get too thick.
Around 10 minutes before the celeriac is ready, carefully open up the foil and start basting every couple of minutes with the melted butter for extra colour.
Drain the pearl barley and dress it with salt, pepper and extra virgin olive oil. Place the celeriac on a board and carve thinly, like you would a joint of meat. Drizzle with any juices from the foil, then serve with the mushroom sauce, pearl barley and lots of beautiful seasonal greens.