Heat olive oil and butter in a large pot over medium heat until melted and hot.
Add the chopped onion and sauté until soft and translucent, about 5 minutes.
Add the minced garlic and cook for an additional minute until fragrant.
Add the sliced mushrooms and cook, stirring occasionally, until they release their juices and become tender, about 8-10 minutes.
Pour in the dry white wine and cook until mostly evaporated, about 3 minutes.
Sprinkle the flour over the mushroom mixture and stir well to combine; cook for 2 minutes to remove the raw flour taste.
Dissolve the chicken bouillon cube in the vegetable broth, then slowly add the broth to the pot, stirring constantly to avoid lumps.
Add salt, pepper, and thyme, then bring the soup to a gentle simmer.
Cook the soup for 15 minutes, stirring occasionally, to thicken and develop the flavors.
Remove the pot from heat and use an immersion blender to puree the soup until smooth. Alternatively, blend in batches in a regular blender.
Return the pureed soup to low heat, stir in the heavy cream, and warm through without boiling.
Taste and adjust seasoning if necessary.
Garnish with fresh parsley before serving.
