Heat olive oil and butter in a large pot over medium heat until butter is melted. Add chopped onion and minced garlic; sauté until onions are translucent and fragrant, about 5 minutes. Add sliced mushrooms and cook, stirring occasionally, until they release their juices and soften, about 8-10 minutes. Pour in the dry white wine and cook until it evaporates, about 2-3 minutes. Stir in the all-purpose flour and cook for 1-2 minutes to form a roux, stirring constantly to avoid lumps. Dissolve the chicken bouillon cube in 4 cups of vegetable broth and slowly add to the pot, whisking continuously to combine. Add fresh thyme, then bring the soup to a gentle boil, reduce heat and simmer for 15 minutes until slightly thickened. Remove from heat and use an immersion blender to puree the soup to your desired consistency, or leave it chunky if preferred. Stir in the heavy cream and season with salt and pepper to taste. Garnish with freshly chopped parsley before serving.