Prepare the mushrooms: Slice the white mushrooms into quarters, then dice them into smaller pieces. Slice the cremini mushrooms similarly.
Sauté aromatics: Melt butter in a large pot over medium-high heat. Add the chopped onion and minced garlic. Cook for 3 minutes until softened.
Cook mushrooms: Add all the mushrooms to the pot. Cook for 10 minutes, stirring regularly.
Simmer the soup: Pour in the vegetable stock, salt, and pepper. Bring to a boil, then simmer gently, uncovered, for 15 minutes.
Add creaminess: Stir in the crème fraîche or cream. Let it simmer for another 5 minutes.
Blend until smooth: Carefully transfer the soup to a blender, working in batches if needed. Blend until completely smooth.
Final simmer: Return the blended soup to the pot. Gently reheat until hot.
Serve: Ladle the creamy mushroom soup into bowls. Garnish with croutons and optional parsley.