In a heavy stockpot, heat the oil over medium-high heat. Add the peppers, potato, onion, salt, and red pepper flakes. Cook, stirring, for 1 to 2 minutes. Reduce the heat to low, cover and simmer, stirring occasionally, until the vegetables are tender, about 1 hour. Remove from heat.
Transfer to a blender or food processor and puree in batches, or puree thoroughly with a hand immersion blender. Force the pulp through a coarse sieve or a food mill. Transfer to a saucepan and stir in the vegetable or chicken stock, adding a bit more if you like a thinner soup. Taste and adjust the seasonings.
Serve warm or cold, garnished with a dab of sour cream and a sprig of dill.