Place the butter in a large pot over medium heat to melt. Add the onions, and stir and cook until they are soft, about 3 minutes. Turn down the heat to low, and whisk in the flour, and let cook 30 seconds. Add the stock, and whisk over low heat until the mixture is smooth. Turn off the heat, and let the mixture cool in the pan 15 minutes.
Stir in the roasted red peppers, and purée the mixture in batches using a food processor orblender. Turn the purée into a mixing bowl, and fold in the sour cream, and season to taste with one of the seasoning blends, or salt and pepper. Cover the bowl with plastic wrap and place in the fridge to chill at least 2 hours, preferably overnight.
Before serving, taste the soup for seasoning, and add more as needed. Spoon the soup into bowls and garnish with croutons and parsley.