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

Toss cauliflower florets with 1 tablespoon olive oil, salt, and black pepper. Drizzle the exposed garlic head with 1 tablespoon olive oil and wrap tightly in foil.

Spread cauliflower across the sheet and place garlic parcel alongside. Roast for 25 to 30 minutes until cauliflower is tender with golden caramelized edges and garlic cloves are buttery and fragrant. Set aside a few pretty roasted florets for garnish.

While vegetables roast, melt butter in a large Dutch oven over medium heat. Add diced onion and celery; sauté for 5 minutes until soft and translucent.

Add cubed potatoes, thyme, and broth. Bring to a boil, then reduce heat to medium-low, cover, and simmer for 15 minutes until potatoes are fork-tender.

Add the roasted cauliflower to the pot. Squeeze the soft roasted garlic cloves directly from their skins into the simmering soup.

Use an immersion hand blender to purée the soup until silky smooth. (Or carefully transfer in batches to a countertop blender, venting the lid with a towel).

Reduce heat to low. Stir in heavy cream, shredded sharp white cheddar, and a pinch of nutmeg. Stir gently until cheese is fully melted and velvety.

Ladle into warm bowls. Top with reserved roasted cauliflower florets, fresh minced chives, cracked black pepper, and toasted croutons.