Place chicken pieces and backbone in a large stockpot. Add carrots, onion, celery, parsley, bay leaf, peppercorns, and 1 tablespoon salt. Add 16 cups water and place over high heat.
When water boils, reduce to a brisk simmer. Skim any foam that accumulates on the surface. Continue to cook, skimming occasionally, until chicken is cooked and broth is slightly reduced, 1 hour.
Remove chicken pieces. Continue to simmer broth. When chicken is cool enough to handle, shred meat into bite-sized pieces, discarding skin and bones. Strain broth into a clean stockpot and discard solids. Skim fat and return chicken to broth. Add desired additions and garnishes, and serve.
TORTILLA SOUP VARIATION:
To hot broth, add corn, tomatoes, black beans, and jalapeno. Maintain a brisk simmer and cook for 2 minutes, until corn and jalapeno are just tender. Add lime juice and chopped cilantro and season to taste with salt and pepper. Serve, garnished with additional lime wedges, cilantro sprigs, tortilla chips, and cheddar cheese.
SHIITAKE-EGG DROP SOUP VARIATION:
To hot broth, add bok choy, shiitakes, scallions, soy sauce, vinegar, and sesame oil. Maintain a brisk simmer and cook for 3 minutes, until bok choy and shiitakes are just tender. Add eggs and stir gently, until set, about 1 minute more. Season to taste with additional soy sauce and vinegar, and serve, garnished generously with sliced scallions.
ASPARAGUS-SPINACH SOUP VARIATION:
To hot broth, add asparagus. Maintain a brisk simmer and cook for 3 minutes, until asparagus is just tender. Remove from heat and add spinach, lemon juice, heavy cream, dill, and basil. Stir until spinach is wilted. Season to taste with salt and pepper and serve, garnished with additional herbs and olive oil.