Preheat the oven to 400°F (200°C). Toss pumpkin chunks and garlic cloves with a tablespoon of olive oil, then spread them evenly on a baking sheet. Roast for about 35-40 minutes until the pumpkin is tender and edges are caramelized.
Meanwhile, heat the remaining olive oil in a large pot over medium heat. Add the chopped onion and cook until translucent and fragrant, about 5 minutes.
Once the pumpkin and garlic are roasted and cooled slightly, peel the garlic and add both pumpkin and garlic to the pot. Cook for a minute to combine the flavors.
Pour in the vegetable broth and bring the mixture to a gentle simmer. Let it cook for about 10 minutes, allowing the flavors to meld.
Use an immersion blender to purée the soup directly in the pot until smooth and velvety. Alternatively, transfer the mixture to a blender in batches and blend until smooth.
Stir in chopped sage leaves and heavy cream for added richness. Season with salt and pepper to taste, then cook for another 5 minutes to heat through.
Adjust seasoning if needed and ladle the hot soup into bowls. Garnish with a sage leaf or a drizzle of cream if desired. Serve immediately.
