Warm the butter and the olive oil in a heavy-bottomed pot. Cook the onions over medium-low heat, without browning and until translucent for about 10 minutes. Add the garlic and cook for another 3 minutes, until fragrant.
Add the cauliflower and season liberally with salt and pepper, (Can use white pepper if preferred), and add 1/2 cup of the vegetable or chicken stock. Cook the cauliflower for 15 minutes or until tender. Lower the heat and bring to a simmer. Add the rest of the stock and cook for additional 20 minutes.
Working in batches, purée the soup in a blender or with an immersion blender until a very smooth, creamy consistency.
Stir in the heavy cream and taste. Adjust seasoning if necessary. If the soup is too thick, add more stock. If its not creamy enough, add a little more heavy cream.
Serve hot, and drizzle with a little extra-virgin olive oil and freshly ground black pepper.