Heat the oil in a saucepan over a medium heat and gently fry the onion for 2-3 minutes, or until softened. Pour in the stock and bring to the boil.
Add the peas and cook for 2-3 minutes. Stir in the cream and two tablespoons of the mint, then blend until smooth using a hand-held blender. Season, to taste, with salt and freshly ground black pepper.
Serve the soup in a bowl, garnished with the remaining chopped mint and the parsley.