Add the butter to a large pot and bring the heat to medium.
Once the butter is melted, add the onions and sauté for 3 minutes, then add the garlic, mushrooms, and chestnuts. Cook for 5 minutes, stirring frequently.
Add the thyme sprigs, broth, soy sauce, salt, and pepper, and bring to a boil. Reduce the heat to simmer and cook for 25 minutes.
Remove the thyme stems. Then, using an immersion blender, blend half of the soup in the pot.
Add in the cream and stir. Simmer for 5 more minutes