Pour the chicken and beef broths into a large saucepan and bring to a boil over high heat. Add the water chestnuts, green onions, and frozen wontons and cook until the wontons are cooked through and floating, 2 to 3 minutes.
Remove from the heat and stir in the fresh spinach. Cover the soup and let the spinach steam and soften in the hot soup for 2 minutes.
Season with Chinese five spice, salt, and cayenne pepper.
