Bring the broth to a steady simmer and keep it simmering.
In a bowl, combine Parmigiano and breadcrumbs. Add the eggs, nutmeg, and salt, and work it into a firm, uniform mixture.
Put some of the mixture into a potato ricer and press it over the pot so strands fall into the broth. Continue until all the dough is used.
When the strands float to the surface, let them cook about 1 minute longer.
Beat the egg yolk with the wine, then stir in the flour until smooth.
Take the pot off the heat and stir in the yolk mixture.
Add parsley and lemon zest and serve immediately.