How to Make It
In a large soup pot, combine the chicken, water, broth, onion, carrot, celery, bay leaf and peppercorns and bring to a boil. Simmer over low heat for 2 hours. Add the cheese rind and simmer for 1 hour. Strain the broth and discard the solids. Skim off as much fat as possible and season the broth with salt. You should have 10 cups.
Meanwhile, in a mini food processor, pulse the mortadella until finely chopped. Add the eggs, egg yolk, lemon zest and nutmeg and process to a loose paste. Transfer the mixture to a medium bowl and stir in the bread crumbs, the 1/4 cup of grated cheese and 1/2 tablespoon of kosher salt; the dough will be stiff. Knead the dough with your hands until the ingredients are evenly combined.
In a large saucepan, bring 4 cups of the broth to a gentle boil. Working in 3 batches, transfer 1/2 cup of the dough at a time to a potato ricer with large 1/4-inch holes. Squeeze 2-inch lengths of the dough into the simmering broth, cutting the dumplings with a paring knife. Repeat immediately with another 1/2 cup of dough. Stir and cook gently just until the passatelli float to the surface and are tender, about 2 minutes. Using a skimmer, transfer the passatelli to a large bowl. Repeat with the remaining dough. Save the broth in the saucepan for another use.
Bring the remaining 6 cups of broth to a simmer. Spoon the passatelli into 6 bowls and top with the pats of butter. Pour the hot broth on top and serve right away, passing grated cheese at the table.