Heat butter and olive oil in a large Dutch oven or soup pot over medium heat.
Add the onions and cook until softened and translucent, about 5 minutes. Stir in the garlic and cook for another minute until fragrant.
Add the flour and cook for another minute.
Cook the Mushrooms
Add the sliced mushrooms to the pot.
At first, the mushrooms will seem like a lot, but they will quickly release moisture and reduce in volume. Continue cooking for 10–12 minutes until they are dark brown and most of the liquid has evaporated.
This step is important because it develops the deep, earthy flavor that makes the soup so delicious.
Add thyme, soy sauce, salt, pepper, and vegetable broth. Also, add shaved Parmesan cheese.
Bring the mixture to a gentle boil, then reduce the heat and simmer for 15–20 minutes.