This is hardly worth its own blog post but making plum jam / puree became the easiest thing in the world once I discovered I could make it in my slow cooker instead of making it in a pot on the stove top! And since it's my favourite jam I usually cook up a big batch when plums are in season and enjoy it all year long! :)
My Slow Cooker has a 3.5 litre capacity so I can stuff in 3 kilos of halved, de-stoned plums cut sides down. It's filled up to the rim and might even touch the lid but that's ok.
Once you've filled your slow cooker with plums, put the lid on and let it cook on high for 3 hours OR on low for 6-7 hours. (I use high when I start cooking in the morning and low when I have it cook over night)
Once that's done take the lid off, stir and keep it cooking on low for up to 24hours or on high for approx. 12-16hours depending on the desired consistency.
DO NOT put the lid back on!
Stir every 2-4 hours.
Add spices. Personally I only add cinnamon (1 teaspoon or so), but you can come up with your own spice mix. Nutmeg, vanilla, cardamom and cloves are some I'd think would be good. Any spice that tastes like Christmas cookies should work just fine.
Make sure the jars are clean and the jam is still hot when filling the jars. Close them right away and let cool completely.
Mine usually keep all year (actually only the last surviving jar) but since the jam doesn't contain any added sugar it doesn't keep well for too long once it's opened.
If you don't have a slow cooker you can also make this recipe on the stove top and in your oven:
Cook the plums on the stove top until they are soft and release liquid. Add spices now if you want to. Then transfer everything to the largest baking pan you have and cook it in the oven for 8-12hours, stirring every 2-3 hours. Make sure to leave the oven a few cm open by sticking in a wooden spoon. Otherwise the released steam can't escape.
Your plum jam is ready when it's got a rich dark colour and rich taste as well as the desired consistency. You're looking for a reduction by at least 50% if not 2/3. My 3kg of plums make approximately 1200-1400ml of jam.