Preheat your oven to 200°C fan. Wash and scrub your celeriac well, removing excess dirt. Using a sharp knife, pierce it all over its whole surface, about one centimetre or so deep. Place the celeriac on a baking tray. In a small bowl, combine your garlic, olive oil, thyme, turmeric and a good seasoning of salt and pepper. Using a pastry brush or your fingertips, cover the celeriac with the oil mixture, spreading the garlic pieces on the celeriac to roast too.
Place in the oven on a low shelf and roast for about two and a half hours. It may take slightly more or less depending on your oven. Check on the celeriac throughout, spooning over any juices that appear in the bottom of the tray.
Once it’s cooked you want a knife to be able to go through easily and for the outside to be a lovely golden brown. Once the celeriac has roasted, remove it from the oven and allow to cool slightly before serving.
While the celeriac is cooking, you can make your gravy. Drizzle a tablespoon or so of oil into a wide pan and start to fry your onions until soft and translucent. Add the mushrooms and garlic and continue to fry for a few minutes – the mixture will become quite moist, due to the mushrooms. Once cooked and fragrant, add about 500 millilitres of the stock, the thyme, rosemary and sage, and stir well. Bring to a gentle boil, then reduce to a simmer.
Add the nutritional yeast, mustard, vinegar, coconut aminos, flour and plant-based milk. Season with salt and pepper to taste. Slowly stir on a low heat to get rid of any lumps, allowing the liquid to thicken. For a thinner gravy, you can go ahead and add the rest of the stock here.
Turn off the heat, then remove a third of the mushroom mixture and put to one side. Either pour the rest of the gravy into an upright blender, or use a hand blender to blitz it until smooth.
Season to taste, then put the blended liquid and the reserved mushrooms back into a saucepan. On a low medium heat, begin to warm the mushroom gravy, stirring to combine.
To serve, place the celeriac, whole, on a serving plate. Score a cross on top and prise it open ever so slightly, pouring over the gravy and allowing it to seep through the gaps. Place the lemon wedges around the dish and season with extra salt and pepper if necessary. You could also plate up individually, cutting the celeriac into ‘steaks’ or any other way you like. Pour over the gravy and serve with lemon wedges, as above.