Heat the butter in a large saucepan, add the leeks and onion, and cook over medium-low heat for 5 to 10 minutes, until the onion is tender.
Add the chicken stock, increase the heat to high, and bring to a boil. Add the peas and cook for 3 to 5 minutes, until the peas are tender. (Frozen peas will take only 3 minutes.) Off the heat, add the mint, salt, and pepper.
Puree the soup in batches: place 1 cup of soup in a blender, place the lid on top, and puree on low speed. With the blender still running, open the venthole in the lid and slowly add more soup until the blender is three-quarters full. Pour the soup into a large bowl and repeat until all the soup is pureed.
Whisk in the creme fraiche and chives and taste for seasoning.
Serve hot with garlic croutons.