Step 1
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.
Step 2
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.
Step 3
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.