Chop the mushrooms – Slice each mushroom into four, then dice into three or four pieces.
Sauté the onion and garlic – Melt butter in a large pot over medium-high heat. Add the chopped onion and minced garlic, cooking for about 3 minutes until softened but not browned.
Cook the mushrooms – Add the mushrooms and sauté for 10 minutes, stirring regularly. No need to brown them, as the pot will be too crowded.
Simmer for 15 minutes – Pour in the vegetable stock, salt, and pepper. Bring to a boil, then reduce to medium heat and let it simmer gently, uncovered, for 15 minutes.
Add cream – Stir in the cream (or crème fraîche) and continue simmering for another 5 minutes.
Blend – Transfer the soup to a blender in batches if needed. Remove the cap from the feeding hole in the blender lid (See Note 5) and cover the opening with a folded tea towel. Blend until smooth.
Final simmer – Return the blended soup to the pot and let it simmer for a minute or two, just until any bubbles from blending subside and the soup is heated through.
Serve – Ladle into bowls and garnish with a drizzle of extra virgin olive oil or cream, croutons, and fresh herbs like parsley or (if you're feeling fancy) chervil. Don’t forget some bread for dunking!