Heat the oil in a stock pot over medium heat. Add in the chopped onion and toss with the oil.
Cook the onion for about 5 minutes until it's translucent and beginning to brown. Stir in the diced garlic and cook for 1 minute.
Add in the chicken stock, water, pumpkin, chipotle pepper and chipotle pepper sauce and stir to combine.
Increase the heat to high and bring the soup to a boil.
Reduce the heat to medium-high and simmer for 10 minutes, stirring occasionally until the soup is beginning to thicken. Remove the pot from the heat.
Use a blender to purée the soup in batches. (Remove the cap from the lid of the blender and cover the opening with a kitchen towel, so that steam can escape and your hand is protected.) Blend each batch for 10-20 seconds to smooth and thicken the soup.
Return the soup to the stock pot. Stir in the salt and pepper.
Serve the soup hot with a small dollop of sour cream stirred in, if desired.
Notes
Chipotle peppers pack a lot of heat, which is great if you like a spicy kick. To dial back the heat use half of a chipotle pepper instead of a whole one.