This seems to come out much stronger than the stuff you buy in jars, and obviously you don't get the same thick syrup with sweetener as you do with sugar, but it will certainly do for cooking purposes.
It's a lengthy process, so don't start just before you want to use it. Get the freshest and youngest ginger you can. It will keep for quite some time, sealed in the fridge.
Large knob of ginger, about the size of your fist
1 cup sweetener
Peel the ginger and slice into 1mm thick slices. Place in a saucepan with the sweetener and enough water to cover. Cook, covered, on a low heat for 2½ – 3 hours, checking on it from time to time to top up the water to stop it boiling dry. The ginger should have a translucent look about it when it's done.