2: Cut the plums in half, remove and discard the stones. Put the fruit into a large bowl.
3: Sprinkle 600 g caster sugar over, mix in briefly, cover and leave overnight.
4: Next day, you will find the plums covered with a syrup – just what we are after.
5: Transfer the plums with their syrup to a large pan on high heat and bring to the boil. Turn down the heat and leave to simmer for 2 minutes.
6: Take off the heat and tip the contents of the pan into a vegetable mill over a bowl...
7: ...and process. You will end up with the plum pulp and juice in the bowl, leaving the skins in the mill.
8: Discard the skins and return the contents of the bowl to the pan on medium heat.
9: Bring to the boil and skim off any scum to keep the jelly nice and clear.
10: In a bowl, mix 40 g jam gelling agent and 100 g caster sugar with a whisk. Note that the correct proportion is 40 g of gelling agent per kilo of fruit+sugar. If you end up with a different weight of fruit+sugar, you can recalculate this amount:
11: Tip this mixture into the pan (still on the boil) and mix in with a whisk.
12: Wait until it comes back to the boil and cook for a further 3 minutes.
13: Pour the jelly into jars.
14: Seal the lids and turn the jars upside down. The air trapped in the jars will be sterilised naturally as it passes up through the boiling hot jam.
15: Turn the jars back the right way up and leave to cool. Store in the dark.
16: Enjoy your plum jelly at your leisure. On a simple slice of toast with a bit of decent butter, it's a real treat.
Remarks
Of course, if you leave the skins in, this will give you a normal plum jam – which is great, too. In this case, I recommend cutting the plums in half or into quarters as this makes the jam easier to spread. All types of plums (Victoria, damsons, mirabelles, greengages...) are suitable for this recipe.