Slice the onion into thin slices using a food processor fitted with a slicing disk. Melt the butter in a skillet, add the onion and saute until limp. Sprinkle the onion with the cumin and saute another minute or two. Add the cooked onion, pumpkin, and salt to the food processor bowl fitted with the metal blade. Process just until the ingredients are blended, about 10 seconds. Add the cream and half and half with the processor running and blend until fully incorporated. Transfer the mixture to a large heavy saucepan. Add the chicken stock and cook over medium-low heat until the soup is steaming but not boiling.

To Serve:
Ladle soup into individual bowls. Drizzle a dollop of crema in the center of each bowl (or a dollop of creme fraiche) then sprinkle with chopped cilantro.