01 - In a large pot, melt the butter (and olive oil, if desired) over medium heat. Add the finely chopped onion and celery; cook until softened, approximately 5 minutes.
02 - Stir in the minced garlic and cook for 1 minute until a fragrant aroma develops.
03 - Add the sliced mushrooms and fresh thyme leaves. Cook, stirring occasionally, until the mushrooms release their natural juices and achieve a golden hue, which should take about 10 minutes.
04 - Season the mushroom mixture with salt and freshly ground black pepper to taste.
05 - Pour in the vegetable broth, bring the mixture to a boil, then reduce the heat and gently simmer, uncovered, for 15 minutes.
06 - Remove the pot from the heat. Using an immersion blender, carefully puree the soup until it reaches your desired smoothness. Alternatively, transfer the soup in batches to a countertop blender and process until smooth.
07 - Stir in the heavy cream and gently heat the soup for 2–3 minutes. Ensure the soup does not come to a boil after adding the cream.
08 - Taste the soup and adjust the seasoning as necessary, adding more salt or pepper if desired.
09 - Ladle the creamy mushroom soup into individual bowls and garnish with fresh chopped parsley before serving.