Chop and pit the plums: if they’re the loose-stone variety, the stones should be easy to yank out when you halve and twist them. The variety I used didn’t budge when I tried this, so I cut around the stone and removed as much of the fruit as possible.
Macerate the plums: Toss the plums with sugar and lemon zest in a bowl. Hands are best for the job. Refrigerate for at least 10 hours (or up to 24 hours). Longer the better.
Add the plums and the juices in the bowl to a blender and coarsely blitz.
Cook in a wide pan over a medium-high heat until the jam thickens and reduces by almost a third. Depending on the width of your pan and the amount of heat, this should take anywhere between 15 - 25 mins.
Stir frequently and skim off any white scum that comes to the top (just take out the bigger clumps that float around the edges of the pan and leave the rest).
A good way to test whether it has thickened and set is to run a spatula through the jam in the pan. If it leaves a clear trail and the jam takes 1-2 seconds to flow back in, it’s done. It will continue to thicken as it cools and also further in the fridge.
Taste the jam after it has cooled. If it’s too sour for your liking, put it back on the heat and stir in some powdered sugar.
Cool completely to room temperature, then decant into bottles and refrigerate. Can be kept refrigerated for up to a month.