Preheat the oven to 180°C / 350°F / Gas mark 4.
Grease a baking dish with butter and set aside.
Place the peeled onions into boiling water, reduce the heat and let them simmer for 5 minutes or longer and just before they are taken out, put in the sage-leaves for a minute or two.
In a food processor, blitz the bread until roughly chopped then add the onions and sage and blitz until very fine, add the seasoning and butter, and work the whole mix together with the yolk of an egg.
It should be highly seasoned, and the sage-leaves should be very finely chopped.
Spoon the stuffing into the baking dish and cook for 20 minutes or until golden on top.