01 - Heat olive oil and butter in a large pot over medium heat. Add chopped onion and sauté for 3 to 4 minutes until softened and translucent.
02 - Stir in minced garlic and cook for 1 minute until fragrant.
03 - Add sliced mushrooms and a pinch of salt. Cook, stirring frequently, for 8 to 10 minutes until mushrooms are golden and most moisture has evaporated.
04 - Add fresh thyme leaves; if using, pour in sherry or white wine. Stir 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 - Use an immersion blender to blend until mostly smooth, leaving some texture if desired. Alternatively, carefully 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.