How to Make It
Melt the butter in a large, heavy pan over medium heat, add the onion and cook for 5 minutes, stirring, until it has softened. Pour in half the stock (broth) and bring to a boil.
Add the peas, shelled and unpodded, reduce the heat and simmer gently until tender (no more than 3 minutes for fresh peas, and about 2 minutes for frozen). Remove from the heat, add the parsley, mint and remaining stock. Transfer the mixture to a blender and purée the soup until as smooth as possible (if you like a very smooth soup, you might also want to strain it through a fine-mesh sieve). Season with salt and pepper and let cool, then refrigerate until really cold.
When you are ready to serve, mix the crème fraîche and double (heavy) cream together in a small bowl. Serve the chilled soup topped with a spoonful of the cream mixture and sprinkled with the chopped herbs.