Mix the breadcrumbs, ¾ cup of the Parmigiano Reggiano, lemon zest, pepper, salt and nutmeg. The mixture should be fine. If it seems too coarse, process it in a food processor until it is almost sandy.
Add the eggs and stir with a spatula until the mixture is blended and feels like sticky dough.
In a large pot, heat the broth. When hot, add the spinach and let the soup simmer for a few minutes to wilt the leaves.
For the following step you can use a potato ricer, or a colander with ⅛-inch holes. If using a ricer, put half of the dough in the ricer and push down directly over the soup so the short strands of dough fall into the simmering soup. Repeat with the rest of the dough. The strands will cook in two to three minutes. If using a colander, place it over the soup pot and use a silicone spatula to push the dough through the holes into the simmering broth. The passatelli are ready when they float to the top.
Serve the soup immediately with extra Parmigiano Reggiano at the table.