Melt the butter at a low temperature in a deep, heavy saucepan. Add the garlic and onions and saute 5 minutes or so until both are soft.
Add the pumpkin, carrots, stock, lemon juice and zest, and sugar to the pot. Stir until thoroughly combined.
Raise the temperature to a medium low level and allow the pot to come to a simmer for about 20 to 25 minutes.
Turn off the heat, and allow the soup to cool.
Puree the mixture in a blender or food processor. You can also smooth it out with an old fashioned potato masher.
Reheat at a low temperature for about 10 minutes.
Season with salt and pepper to taste.
Just before serving, stir in a dash or two of cream, and add the chives.