Tip the porcini into a measuring jug. Add 800ml boiling water and set aside to soak.
Peel and finely chop the onion and carrot. Trim and finely slice the celery. Warm a large pan for 1 min, then add the chopped veg and 1 tbsp oil. Season. Cover, keep the heat low and sweat for 8 mins till the veg have softened a little.
Slice the mushrooms. Peel and finely chop the garlic. Add them to the pan. Season with a pinch more salt and pepper. Put the lid back on and sweat the veg for a further 5 mins till the mushrooms look juicy.
While the veg sweats, peel and coarsely grate the beetroot. Stir in the beetroot. Add the porcini and its soaking liquid to the pan. Add 1 bay leaf and most of the thyme. Cover, turn the heat up and bring to the boil. Turn the heat down a little and simmer for 10 mins till the veg are very tender.
Taste and season if needed. Ladle into warm bowls, discarding the bay leaf and thyme sprigs. Garnish with a few thyme leaves to serve.