Preheat oven to 400°F (200°C). Line a large baking sheet with parchment paper for easy cleanup. In a large bowl, toss the cubed butternut squash with 2 tablespoons of olive oil, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Spread evenly on one half of the prepared baking sheet.

Place the head of garlic (with the top sliced off) on the other half of the baking sheet. Drizzle the garlic with the remaining 1 tablespoon of olive oil. You can wrap it loosely in foil if you prefer softer, more contained garlic.

Roast for 25-30 minutes, or until the squash is tender and lightly caramelized, and the garlic cloves are soft and golden brown. Remove from oven and let cool slightly.

While the squash is roasting, heat 1 tablespoon of olive oil (from the divided amount, if not already used) in a large stockpot or Dutch oven over medium heat. Add the chopped onion and sauté for 5-7 minutes, until softened and translucent.

Add the chopped fresh thyme to the pot and cook for another minute until fragrant.

Once the roasted garlic is cool enough to handle, carefully squeeze the softened cloves from their skins into the pot with the sautéed onion. Add the roasted butternut squash to the pot.

Pour in the vegetable broth and add the ground nutmeg. Bring the mixture to a simmer, then reduce heat to low, cover, and let it cook for 10 minutes to allow the flavors to meld.

Remove the pot from the heat. Using an immersion blender, carefully blend the soup directly in the pot until it is completely smooth and creamy. If you don't have an immersion blender, transfer the soup in batches to a standard blender (be cautious with hot liquids – fill the blender only halfway, vent the lid, and cover with a kitchen towel) and blend until smooth. Return the blended soup to the pot.

Stir in the heavy cream (if using). Taste and adjust seasoning with additional salt and black pepper as needed.

Serve hot, garnished with toasted pumpkin seeds and fresh thyme sprigs, if desired.