This is an old family recipe, my family used white onions but I prefer red!

4 large red onions, peeled (or sweet white onions if you prefer)
8 fresh sage-leaves – or to taste
¼ lb of fine white breadcrumbs
2 oz softened unsalted butter
Salt and white pepper
Yolk of 1 large egg

Put the onions into a saucepan, add sufficient boiling water to cover and allow them to simmer gently until soft. Put the sage leaves in for a few seconds before cooking has finished to blanch
If fresh sage is not available, dried sage is a good alternative, but use sparingly
Drain (retaining cooking liquid), chop onions and sage leaves finely (or blitz if you prefer, add bread, butter and seasoning to taste. Add the egg yolk and mix thoroughly to combine. If the mixture is too dry, add a little of the onion water.
This stuffing should be highly seasoned and the sage-leaves should be very very finely chopped.

Notes:

This stuffing can be either made into balls and cooked around the bird or stuffed into the neck end of the turkey
When I was young, a little was added to the individual Yorkshire Puddings on Christmas Day, these were traditionally eaten as a starter with onion gravy
Adding liver to the stuffing is also popular, but I prefer it without
Mum used the “onion water” to de-glaze the roasting tin and made a traditional gravy using the meat juices