About 12 cups (about 6 lbs) pitted and roughly chopped plums
1 or 2 green apples
Juice of 1 lemon or lime
2 lbs. granulated sugar
2 small plates in the freezer
Cut up all the plums and put them into a heavy, 5-quart stockpot.
Grate the apple, skin and all (but not the core), on a box shredder-grater. Add the apples, lemon or lime juice, and sugar to the plums.
Stir the plum mixture thoroughly, place over medium-high heat, and bring to a boil.
Lower the heat to medium and let boil, skimming the foam occasionally and smashing the fruit as it cooks. Boil for about 20 minutes.
When the jam starts to look thickened, start testing it by spooning a small amount onto one of the chilled plates. This will chill it quickly and let you see how thick it is. Keep testing it until the jam is thick enough, but don't cook it longer than about 30-35 minutes. If it's not thick enough for your liking, next time add an extra apple. Don’t worry, the jam will still be great.
Once the jam has thickened, get your clean jars set up. You can re-use the glass jars, but you should get new lids and rings each time. If you have a canning funnel, it makes your life a lot easier for filling jars.