Preheat your oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper.

Toss cauliflower florets with 2 tablespoons olive oil, salt, and pepper. Spread evenly across the baking sheet. Place the garlic head cut-side up on foil, drizzle with remaining 1 tablespoon olive oil, wrap tightly, and nestle beside the cauliflower.

Roast for 30 to 35 minutes until the cauliflower florets are deeply golden-brown and caramelized at edges, and garlic cloves are soft and sweet. Reserve 1/2 cup roasted florets for garnish.

Melt butter in a large heavy Dutch oven or soup pot over medium heat. Add diced onion and sauté for 5 minutes until soft and translucent.

Squeeze the roasted garlic cloves out of their papery skins directly into the pot. Add roasted cauliflower, fresh thyme, and chicken or vegetable bone broth. Bring to a gentle boil, then simmer for 10 minutes.

Remove from heat. Use an immersion blender (or carefully transfer to a high-speed blender in batches) to puree until completely smooth, glossy, and velvety.

Stir in heavy cream, grated Parmesan, nutmeg, and fresh lemon juice over low heat until warm. Ladle into bowls, top with reserved roasted florets, a drizzle of olive oil, and serve!