Heat the beef marrow over low heat, melt, pour through a sieve and stir with a whisk until foamy. Add in the egg, bread crumbs, lemon zest, nutmeg and parsley, and season with salt. Let rest for 30 minutes.
Make small balls out of the mixture and drop into slightly salted boiling water. Cook for about 15 minutes until done. Then drain.
Use the dumplings in soups.