In the microwave safe pan, cook the chopped plums, sugar, and lemon juice on HIGH for 7 minutes, stirring twice during cooking.
Add the chopped ginger, stir, and cook on HIGH for 15 to 18 minutes, stirring every 4 minutes. The mixture should form a gel.
Ladle into hot jars, leaving ¼-inch headroom. Process for 10 minutes in the prepared boiling water canner. Or you can freeze the jam if desired. Will keep unprocessed in the refrigerator for about 3 weeks.
You can also make this on the stove top using a 2 quart/liter sauce pan. Add the chopped plums, sugar, and lemon juice to the pan, stir, and bring to a boil. Stir frequently. Add the crystallized ginger, stir again, and bring back to a boil. Reduce heat to a simmer and cook until the mixture starts to gel, about 15 to 20 minutes. Continue with Step 4.