Heat olive oil and butter in a large pot over medium heat. Add diced onions with a pinch of salt and sauté for 2-3 minutes until they soften. Next, toss in the mushrooms in two batches, allowing the first batch to cook down a bit before adding the rest. Stir occasionally for 10 minutes, letting the mushrooms release their juices and turn golden-brown.

Stir in the garlic and cook for 30 seconds, until it smells fragrant. Deglaze the pot by adding white wine, scraping up any bits stuck to the bottom. Pour in vegetable broth, tamari, and the thyme sprigs. Stir everything gently, bring it to a boil, then reduce the heat to a simmer. Allow the soup to cook covered for 15 minutes, letting the flavors meld beautifully.

Once the soup has cooled slightly, puree about two-thirds of it until smooth using an immersion or regular blender. Stir the blended mixture back into the pot for a perfect balance of creamy and chunky textures. If you want an indulgently creamy finish, add heavy cream or crème fraîche at this point. Give it a final season with salt and pepper before serving piping hot.