Prepare all the necessary components.
Wash the plums thoroughly, cut them in half and remove the pits.
Place the plums in a cooking pot, add water and steam under a lid until soft.
Rub the hot puree through a colander.
Wash the apples thoroughly, peel, remove the seeds and cut into slices.
Place the apples in a stainless steel container, cover with a lid and cook until softened.
Pass the hot puree through a colander again.
Mix the plum mixture with the apple mixture.
Cook until thickened, stirring thoroughly.
Add granulated sugar. Boil the jam until ready.
To check for doneness, drop a small amount of the mixture onto a cold glass saucer. If the drop doesn't spread and holds its shape, and hardens upon cooling, stop cooking. Cooking time for the jam should not exceed 45 minutes.
Pour the prepared hot jam into sterile jars, but do not close them; leave until it hardens and a crust forms on the surface.
Close the jars tightly and keep them in a cool, dry place.