Preheat your oven to 400°F (200°C). Spread the peeled, seeded, and cubed butternut squash evenly on a baking sheet, then drizzle with olive oil and toss to coat. Roast for about 25-30 minutes until the squash is golden and tender.
While the squash roasts, heat a large pot over medium heat. Add a splash of olive oil, then sauté the chopped onion until it becomes translucent and fragrant, about 5 minutes.
Add the minced garlic to the onion and cook for another minute until it releases a fragrant aroma.
Remove the roasted squash from the oven and add it to the pot with the sautéed onion and garlic.
Pour in the vegetable broth, then bring the mixture to a gentle simmer. Let it cook for about 10 minutes to meld the flavors.
Use an immersion blender or transfer the soup carefully to a blender, then blend until completely smooth and velvety.
Return the pureed soup to the pot if you used a blender, then stir in the heavy cream if desired for extra richness. Season with salt and pepper to taste.
Warm the soup over low heat until it reaches your preferred serving temperature, about 5 minutes, stirring occasionally.
Serve hot, garnished with toasted seeds for added texture and flavor.
