Melt the butter in a saucepan over medium heat. Add the onion and green and red chillies and cook for 5 minutes, or until softened. Increase the heat to high, add the garlic and paprika, and cook for 1 minute, or until fragrant.
Add the beer, bring to the boil and cook until almost evaporated. Reduce the heat to low and add the sour cream, stir