Take a large pot. Melt the butter over medium heat. . Add the onions and cook, stirring occasionally, until the onions are lightly caramelized, about 5 minutes. If the onions begin browning too quickly, turn the heat to low.
Add the garlic, sugar, cinnamon, pepper, nutmeg and cayenne pepper and cook for 3 minutes.
Add the pumpkin puree and broth, stir to incorporate, season with salt and bring to a boil. Reduce the heat to low and simmer for 20 minutes.
Let cool for 10 minutes.
Transfer the soup to a blender (or use an immersion blender in the pot) and puree until smooth. Add more broth if the soup is too thick.
Serve warm and drizzle with crème fraîche. You can sprinkle parsley on top.