In a large heavy pot with high sides, stir together the chopped plums, water, lemon juice and sugar. Bring to a simmer over low heat, stirring frequently, until the sugar dissolves.
Increase heat to high, and bring to a hard boil. Cook, stirring frequently, for about 25 minutes. Check for set.
Once the mixture passes a set test, stir in the grated ginger and Averna. Continue cooking for 5 minutes, stirring constantly, then remove from heat and ladle into clean, sterilised jars, leaving ¼" headspace.
Jam can be stored in the fridge for up to 6 months. For longer storage, process jars in a hot water bath for 10 minutes and store the sealed jars in a cool, dry place for up to a year.
Recipe by Crumb: A Food Blog at https://www.crumbblog.com/we-be-jammin-gingery-plum-jam/