Slice the plums in half and remove the pips.
Roughly chop the onion and peel the garlic.
Combine all of the ingredients together in a large pot or jam pan and bring to the boil.
Once the mixture has reached a boil, reduce the heat and allow the pulp mixture to reduce. Stirring often to avoid the mixture from sticking.
Once the sauce has reduced use a stick blender to make a smooth sauce.
Bottle the sauce while it is hot, and use a funnel to pour it into hot sterile bottles. Please be careful and take care not to burn yourself.
Use a clean cloth to clean the rims before placing the lids on the bottles.
Leave the bottle to seal overnight. There is nothing more satisfying than hearing the lids go ‘pop’ signifying that they have sealed.
Once the bottles have cooled and are sealed wash them in hot soapy water to get rid of any sticky spots.