Place a large, wide pot over medium heat.
Add the mushrooms, toss to coat in the oil, then leave them undisturbed for 5 minutes.
After 5 minutes, toss around and saute for an additional 5-8 minutes, continue to toss as needed. Then transfer the mushrooms to a plate.
Add the onion to the pot, and a splash of oil, if needed. Saute for 2 minutes, then toss in the garlic and cook for 60 seconds. Season with salt and pepper.
Pour in broth, the bundle of thyme and mushrooms. Bring to a boil and simmer for 15 minutes.
Take out 2 cups of the whole soup (with everything in it) and blitz with walnuts and miso, until creamy. Return to pot. Remove the thyme. Reason with salt and pepper. Simmer for an additional 10 minutes.
Pour into bowls, drizzle with olive oil and springs of fresh thyme.