Melt the butter in a saucepan, add the onion and potato and cook for 5 minutes. Add the stock and bring to the boil, then reduce heat and simmer gently for 10 minutes or until the potato is tender.
Add the peas to the pan and cook for a further 3–4 minutes. Season well with salt and pepper, remove from the heat and stir in the mint. Purée in a food processor or blender until smooth. Ladle into warmed bowls and top each with a dollop of crème fraîche, if liked.