Thinly slice the spring onions, reserving some of the green tops for garnish. Heat the butter, fry the spring onions for 1 min until slightly softened. Add the peas and stock, then bring to the boil.
Simmer for 5 mins, then whizz half the soup in a food processor. Return to the pan with the crème fraîche. Reheat gently, taste and add pepper, and salt if it needs it. Ladle into small bowls or cups and saucers, then sprinkle with the reserved spring onion tops.