Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Toss the cauliflower florets and peeled garlic cloves with 1 tablespoon of olive oil until evenly coated. Spread them out on the prepared baking sheet in a single layer.
Roast the vegetables in the oven for about 25-30 minutes, until the cauliflower is golden and tender, and the garlic is soft and fragrant.
Meanwhile, melt the butter in a heavy-bottomed pot over medium heat. Add the diced onion and cook for about 5 minutes, stirring occasionally, until translucent and fragrant.
Once the cauliflower and garlic are roasted, add them to the pot. Use a spoon to scrape up any caramelized bits from the baking sheet for extra flavor.
Pour in the vegetable broth, then bring the mixture to a gentle simmer. Let it cook for 10 minutes to meld the flavors.
Use an immersion blender or transfer the soup in batches to a blender. Purée until silky smooth, ensuring an even, velvety texture.
Return the puréed soup to the pot if needed, and season with salt and pepper to taste. Warm through for a few minutes, stirring occasionally.
Serve the soup hot, garnished with fresh herbs or a drizzle of olive oil if desired, and enjoy its warming, creamy goodness.