Boil the chestnuts in a pot of water for 20 minutes, then drain and set aside.
In a skillet, sauté the onion and garlic in margarine over medium-high heat until lightly browned and aromatic.
Add the boiled chestnuts and fry for another 2 minutes.
Pour in white wine vinegar and enough vegetable broth to cover the chestnuts. Add oat milk, cover, and simmer for 10 minutes.
Season the soup with salt and black pepper to taste. Blend the soup until smooth and creamy.
In a separate pan, sauté the mushrooms in olive oil until golden brown. Season with salt, pepper, and thyme. Add to the soup for garnish.