Heat butter in a large saucepan, add the leeks and onion, and cook over medium-low heat for 5 to 10 minutes, until onion is tender. Add the chicken stock, increase heat to high, and bring to a boil. Add the peas and cook for 3 to 5 minutes, until peas are tender. (Frozen peas will take only 3 minutes.) Remove from heat; add the mint, salt, and pepper.
Place 1 cup of soup in a blender, place lid on top, and puree on low speed. With blender still running, open the vent hole in the lid and slowly add more soup until blender is three-quarters full. Pour pureed 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. Makes 6 servings.
Place chickens, onions, carrots, celery, parsnips (if using), parsley, thyme, dill, garlic, kosher salt, and peppercorns in a 16- to 20-quart stockpot. Add water and bring to a boil. Simmer uncovered for 4 hours. Strain the entire contents of the pot through a colander and discard the solids. Pack in quart containers and chill overnight. Refrigerate for up to 5 days or freeze for up to 6 months.