As you will have noticed, I have varied from the original recipe by not parboiling all the ingredients together, but instead have sautéed the onion & parsley and then mixed with the other items. My reason for this is personal: I have made the boiled version before, and simply do not like it! Feel free to boil the stuffing as described, but be assured that the modern recipe works just as well (if not better), and uses the same cooking procedure indicated in other, similar medieval stuffing recipes.
I've included both the onions & the grapes in the modern recipe, but using either one by itself is perfectly fine.
Take parcill, Swynes grece, or suet of shepe, and parboyle hem in faire water and fressh boyling broth; And žen take yolkes of eyeron hard y-sodde, and hew hem smale, with the herbes and the salte; and cast therto pouder of Ginger, Peper, Canell, and salte, and Grapes in tyme of yere; And in ožer tyme, take oynons, and boile hem; and whan they ben yboiled ynowe with že herbes and with že suet, al žes togidre, žen put all in že goos, or in že Capon; And then late him roste ynogh.
Take parsley, swine's grease, or suet of sheep, and parboil them in water and fresh broth; And then take hard-boiled egg yolks, and cut them small, with the herbs and the salt; and add powder of ginger, pepper, cinnamon, and salt, and grapes in time of year; And in other times, take onions, and boil them; and when they have boiled enough with the herbs and with the suet, all these together, then put all in the goose, or in the capon; And then let it roast enough.
Austin, Th.: Two Fifteenth-Century Cookery-Books; Gode Cookery