01 - Heat olive oil and butter in a large pot over medium heat. Add chopped onion and cook for 3 to 4 minutes until softened and translucent.
02 - Incorporate minced garlic and cook for 1 minute until fragrant.
03 - Add sliced mushrooms and a pinch of salt. Sauté for 8 to 10 minutes until golden and most liquid has evaporated, stirring often.
04 - Sprinkle fresh thyme leaves and, if using, pour in sherry or white wine. Cook for 1 to 2 minutes until the liquid evaporates.
05 - Pour in vegetable stock. Bring to a simmer, then reduce heat and cook uncovered for 10 minutes.
06 - Use an immersion blender to purée the soup until mostly smooth, retaining some texture, or blend carefully in batches using a countertop blender.
07 - Return soup to low heat, stir in heavy cream, and warm through gently. Season with salt and black pepper to taste.
08 - Ladle soup into bowls, garnish with additional thyme and parsley, and serve hot.