Preheat the oven and roast the pumpkin chunks on a baking sheet until golden and tender, about 40-45 minutes, which makes peeling and pureeing easier.
While the pumpkin roasts, heat olive oil in a skillet over medium heat. Add sliced onions and cook slowly, stirring occasionally, until caramelized and fragrant, about 15-20 minutes.
Once the pumpkin is cool enough to handle, peel the skin if needed and transfer the pumpkin flesh to a blender or use an immersion blender directly in the pot. Add the roasted pumpkin chunks, caramelized onions, minced garlic, cinnamon, nutmeg, and vegetable broth.
Blend everything until smooth and velvety. Pour the mixture into the slow cooker and set on low for 6-8 hours, allowing flavors to meld deeply and develop richness.
Once the cooking time is complete, open the lid and stir the soup. For a creamier texture, add heavy cream and season with salt and pepper to taste. Let it heat through for about 10 minutes.
Serve the soup hot, garnished with a drizzle of cream, a sprinkle of cinnamon, or a few pumpkin seeds for added texture and visual appeal.
