This is really easy, so easy that even my Mum could make it.
1kg red plums, pitted and diced
1 large onion, roughly chopped
1 apple, peeled, cored and diced
250g brown sugar
2 dried red chillies, flaked (keep seeds in if you want it hot)
1 tsp salt
2 tsps yellow mustard seeds
250ml apple cider vinegar
250ml red wine vinegar
1 cinnamon stick
Place all of the ingredients into a suitable pan and bring to a boil. Turn down the heat and leave to simmer for an hour so.
Stir gently from time to time.
When cool pour into sterilised jars and keep in a cool dark place or the refrigerator.