Preheat your oven to 200°C (400°F). Place the small sugar pumpkin on a roasting pan and roast for about 45 minutes until the flesh is deeply caramelized and very tender when pierced with a fork.
Carefully scoop out the roasted pumpkin flesh, avoiding any charred bits that add smoky depth. Set aside.
In a heavy-bottomed pot, melt the butter over medium heat. Add the chopped onion and cook, stirring frequently, until translucent and fragrant, about 5 minutes.
Add the minced garlic to the pot and cook for another minute until it becomes aromatic, being careful not to burn it.
Stir in the roasted pumpkin flesh, then splash in the apple cider vinegar. Sprinkle the cinnamon over the mixture and cook for 5 minutes, stirring occasionally, to combine the flavors.
Pour in the vegetable broth until the pumpkin mixture is mostly covered. Bring the mixture to a gentle simmer and cook for 10 minutes to meld the flavors.
Use an immersion blender to carefully blend the soup directly in the pot until completely smooth and velvety. Alternatively, transfer the mixture in batches to a blender and purée until silky.
Return the blended soup to low heat. Stir in the heavy cream and season with salt and pepper to taste. Warm through for another 10 minutes, but do not boil.
Taste and adjust the seasoning if needed, adding more vinegar or cinnamon for brightness or warmth.
Serve hot, garnished with a swirl of cream or a sprinkle of cinnamon if desired. Enjoy the smoky, velvety richness of this fall-inspired pumpkin soup.
