Very very easy. It goes wonderfully with lamb and beef. It also goes very well with a strongly flavoured cheese, like a good cheddar.
This will make a small bowl's worth, depending on the size of the onions.
Peel and dice the onions. Heat the oil in a pan. Once the oil is hot add the onions to the pan.
Add the sugar, the raisins and the vinegar to the pan.
Once the liquid is bubbling lower the heat as low as you can and leave it for 20mins to reduce the liquid and let the flavor come of of the onions.
Take off the heat and tranfer to a glass bowl. Put in fridge. Use as required.