In a large pot or Dutch oven, heat the olive oil and butter over medium heat until melted.
Add the diced onion and sauté for about 5 minutes until it becomes translucent.
Stir in the minced garlic and cook for another minute until fragrant.
Add the sliced mushrooms to the pot, and sauté until they release their moisture and become golden brown, about 10 minutes.
Pour in the vegetable broth, then add the dried thyme, salt, and black pepper. Bring the mixture to a simmer.
After simmering for about 10-15 minutes, remove the pot from heat. Using a blender or immersion blender, carefully blend the soup until it reaches your desired level of creaminess.
Return the blended soup to the pot and stir in the heavy cream. Heat over low for an additional 5 minutes, making sure not to boil.
Taste and adjust seasoning if necessary. Serve hot, garnished with fresh parsley.