Heat the remaining 1 tbsp of oil in a stock pot over a medium heat and fry the onion for 10 minutes until tender but not golden. Add the roasted celeriac, scraping in any roasting juices from the tray. Squeeze the roasted garlic from its skins into the pot but discard the thyme sprigs. Add the miso and stock, bring to a simmer and cook for 10-15 minutes. Blitz the mixture with a hand blender until very smooth – strain through a sieve into a clean pan to ensure it’s completely smooth, if you like.