Preheat the oven to 200°C/fan 180°C/gas mark 6. Place the whole garlic bulb with seasoning in a piece of tin foil and wrap tightly. Place on a baking tray and bake for 45 minutes. Once cooked and very soft to the touch, leave out to cool.
Add the porcini mushrooms to 1¼ litres of boiled water and leave to soak for 20 minutes. Save the porcini for another use and keep the mushroom stock on a low heat.
Heat half the olive oil in a large heavy bottomed pan on a medium heat. Add the mushroom mince and thyme leaves, season and cook for 8 minutes until browned. Transfer to a bowl and set aside.
Add the remaining olive oil to the same pan. Once hot, add the onions and sweat for 5 minutes until soft but not coloured. Add the rice to the pan and stir for 2 minutes to coat. Increase the heat slightly and add the sherry, letting it bubble away until almost completely reduced.
Add the first ladle of stock, stirring often. When it has been almost all absorbed, add another ladle. Continue for 20 minutes until the rice is cooked al-dente. Remove the pan from the heat. Add the cooked mushroom mince, vegan cheese and seasoning and mix well. Transfer to a baking dish and set aside to cool uncovered for 1 hour, then cover and transfer to the fridge for at least 4 hours.
Meanwhile, squeeze the flesh out of the skin of the garlic bulb. Place in a bowl with the yogurt, lemon juice and olive oil. Mix to combine and season to taste.
Place the soya milk and soft vegan cheese in a bowl and beat with a fork until combined. Add the panko breadcrumbs to a second bowl. Form a ball of cold risotto in your hands, dip in the wet mix and cover with the breadcrumbs. Repeat to make 22 arancini. Transfer to a lined baking tray and chill in the fridge for 30 minutes.
Heat the vegetable oil in a heavy-based deep saucepan to 170°C. Add 4-5 arancini to the oil and fry until golden and crisp all over. Remove from the oil with a slotted spoon and place on kitchen paper. Mix the chives through the dipping sauce and serve alongside the arancini.