Preheat your oven to 400°F (200°C). Cut the butternut squash in half, scoop out the seeds, and cube it into even pieces.

Spread the cubes on a baking sheet, drizzle with olive oil, and sprinkle with salt. Roast in the oven for about 30 minutes, or until the squash has caramelized and is fork-tender.

In a large pot, melt 3 tablespoons of butter over medium heat. Once melted, add the diced onion. Sauté for about 5 minutes, or until the onion is soft and translucent.

Add the roasted butternut squash to the pot, along with the vegetable broth. Stir to combine, and bring the mixture to a gentle simmer. Let it simmer for about 15 minutes, allowing the flavors to meld together.

Using an immersion blender, blend until the soup is smooth and creamy. If using a countertop blender, do so in batches and return to the pot afterward.

Stir in the heavy cream and season with salt and pepper to taste. Heat gently until warmed through, making sure not to bring it to a boil.

In a small skillet, heat a bit of butter over medium heat. Fry the fresh sage leaves until crispy, which should only take a couple of minutes. Serve the soup topped with crispy sage leaves for garnish.