Prepare Mushrooms: Slice each mushroom into four parts, then further cut these slices into 3 or 4 smaller pieces.
Onion and Garlic Sauté: In a pot over medium-high heat, melt the butter before adding onions and garlic, cooking them until they're just softened, ensuring they don't turn golden.
Mushroom Cooking Phase: Introduce the mushrooms to the pot, stirring them frequently over a 10-minute period. They should soften without browning due to the volume in the pot.
Stock Simmer: Add the stock along with a pinch of salt and pepper, bring to a boil, then reduce the heat to maintain a gentle simmer for 15 minutes without a cover.
Cream Integration: Incorporate the cream or crème fraîche into the pot, allowing it to simmer for an additional 5 minutes to marry the flavors.
Smooth Blend: Shift the soup mixture to a blender, taking care to remove the lid's cap and secure it with a kitchen towel over the opening, then blend until the texture is completely smooth.
Heat Through: Pour the soup back into the pot, simmering briefly to ensure it's thoroughly heated and the surface bubbles have dissipated.
Serve and Enjoy: Dish the soup into bowls, embellishing each serving with a drizzle of cream or olive oil, a sprinkle of croutons, and a garnish of parsley, chervil, or thyme leaves. Offer bread on the side for the ultimate dunking experience.