Heat the butter and olive oil in a large saucepan over medium heat.
Fry the onion for about 10 minutes until translucent and golden, stirring occasionally.
Add the corn and chili pepper and continue frying about 5 minutes, again stirring occasionally.
Add the stock, season with salt and pepper and bring to the boil. Simmer for 10 minutes.
Transfer 2/3 of the soup to a food processor and whiz to a coarse puree. Add back into the pan and stir in the lemon juice and parsley. Taste for seasoning.
Serve with a spoonful of sour cream and more parsley, if desired.