Preheat your oven to 425°F (220°C). Cut the top off the garlic head to expose the cloves, drizzle with 1 tablespoon of olive oil, then wrap it in foil. On a baking sheet, toss the cauliflower florets with the remaining olive oil, salt, and pepper. Arrange the cauliflower and the wrapped garlic on the sheet and roast for about 25-30 minutes, until the cauliflower is golden brown and the garlic cloves are soft and caramelized.
While the garlic and cauliflower are roasting, heat a large pot over medium heat. Add a splash of olive oil and sauté the diced onion for about 5-7 minutes, or until translucent and fragrant.
Once the garlic is cool enough to handle, squeeze the softened cloves from their skins into the pot with the onions. Add the roasted cauliflower and pour in the vegetable broth. Bring the mixture to a boil, then reduce the heat and let it simmer for 15 minutes to allow flavors to meld.
Using an immersion blender, puree the soup directly in the pot until smooth and creamy. Alternatively, carefully transfer the soup in batches to a blender and blend until silky. Return the soup to the pot if needed.
Pour in the heavy cream and stir to combine. Taste and season with salt and pepper as desired. Heat gently until warmed through, but avoid boiling to keep the cream from curdling.
Ladle the soup into bowls and garnish with freshly chopped chives. This soup pairs beautifully with crunchy bread or crispy roasted vegetables for extra texture.
