Preheat the oven to 190°C.
Wash the plums, remove the pits, and cut them into quarters. Place the prepared fruit into a deep baking dish or roasting tin.
Put the dish in the preheated oven and bake for 30 minutes. After this time, carefully remove the dish from the oven (watch out for the hot steam) and stir the plums; they should have released a lot of juice. Check the sweetness. If needed, add a tablespoon of sugar (or more) and mix well. Return the dish to the oven for another 30 minutes.
After this, take the dish out of the oven and check the consistency (the jam will thicken further as it cools). If not all the plums have broken down, you can mash them with a spoon. If the jam is still too runny, put it back in the oven for an additional 15-20 minutes (be careful not to over-bake). Transfer the finished jam into sterilised jars and process them in a pot of water. Done!