01 - Heat olive oil and butter in a large pot over medium heat. Add the chopped onion and cook for 3 to 4 minutes until softened and translucent.
02 - Add minced garlic and sauté for 1 minute until fragrant.
03 - Add sliced mushrooms and a pinch of salt. Cook, stirring often, for 8 to 10 minutes until mushrooms turn golden and most liquid evaporates.
04 - Stir in fresh thyme leaves. If using, add sherry or white wine and cook for 1 to 2 minutes until liquid mostly evaporates.
05 - Pour in vegetable stock, bring to a simmer, then reduce heat and cook uncovered for 10 minutes.
06 - Using an immersion blender, puree the soup until mostly smooth, leaving some texture. Alternatively, blend in batches in a countertop blender.
07 - Return soup to low heat, stir in heavy cream, and warm gently. Season with salt and freshly ground black pepper to taste.
08 - Ladle soup into bowls, garnish with extra thyme and parsley, and serve hot.