Preheat oven to 400°F (200°C). Peel, deseed, and cut the butternut squash into 1-inch cubes. Chop the yellow onion, mince the garlic, and chop the fresh sage.

On a large baking sheet, toss the cubed butternut squash with 2 tbsp olive oil, 1/2 tsp salt, and 1/4 tsp black pepper. Spread in a single layer. Roast for 25-30 minutes, or until fork-tender and slightly caramelized.

While the squash roasts, melt 2 tbsp unsalted butter in a large pot or Dutch oven over medium heat. Add chopped yellow onion and cook until softened and translucent, about 5-7 minutes. Add minced garlic and chopped fresh sage (or dried sage) and cook for another minute until fragrant.

Add the roasted butternut squash to the pot with the sautéed aromatics. Pour in the 4 cups of vegetable broth, then stir in the 1/4 tsp ground nutmeg and pinch of cayenne pepper (if using). Bring the mixture to a simmer.

Reduce heat to low, cover, and let it simmer for 10-15 minutes to allow the flavors to meld.

Remove from heat. Carefully transfer the soup to a blender in batches, or use an immersion blender directly in the pot, blending until completely smooth and creamy.

Return the blended soup to the pot (if using a standing blender). Stir in the 1/2 cup heavy cream. Heat gently over low heat, do not boil. Taste and adjust seasoning with additional salt and black pepper as needed.

Ladle the hot soup into bowls. Garnish with toasted pumpkin seeds, a swirl of cream, or crispy fried sage leaves, if desired. Serve warm.