Heat olive oil in a large heavy saucepan over medium heat. Add onion and saute until soft, 8 to 10 minutes.
Add stock, red peppers, tomatoes, sugar and basil. Crush tomatoes roughly with a wooden spoon. Bring to a boil, then reduce heat, cover and simmer for 15 minutes.
Remove tomatoes and peppers with a slotted spoon and transfer to food processor. Blend until smooth , then return to the liquid. Season with salt and pepper to taste. Serve each bowl with a dollop of sour cream, if desired.