Heat the oil in a large saucepan, add the onion and gently cook for 3 minutes until softened. Add the peas and stock, bring to the boil, reduce the heat and simmer for 3 minutes. Add the mash, bring back up the boil and simmer for 2 minutes.
Season with black pepper and stir in the mint and half the crème fraîche. Pour the soup into a blender (or use a stick blender) and whizz until smooth. Ladle into warmed bowls and swirl in the remaining crème fraîche.