What I was after here was a soft-set jam, sweet enough to counter the lip-puckering tartness of the plums while still retaining its original flavour (this recipe is versatile, so you can add more sugar if you want). I also wanted it to look and taste as homemade as possible, so I kept the fruit quite chunky and left the skins on. The only drawback with using less sugar is that it won’t keep for as long – two months maximum, but I seriously doubt that that would be a concern.
Note 1: I like to keep the fruit quite chunky. If you’d prefer it smooth, purée the plums after cooking them (before you add the sugar).
Note 2: The jam will thicken slightly as it sits in the refrigerator.
Place a heavy-bottomed pot over medium heat and toss in the plums. Stir, then clamp on a lid and cook until tender and the plums are falling apart, about 5-10 minutes (the timing will depend largely upon the ripeness of the plums). Push the plums down using a spatula and mush them as you go along.
Once the plums are tender, add the sugar to the pot. Cook the jam over medium-high heat stirring frequently until the sugar is dissolved. Continue simmering for a further 10 minutes, skimming off any scum that rises to the surface (note: stir frequently at this stage as there is a risk of the sugar catching at the bottom).
Continue cooking until the jam has thickened, another 5 - 7 minutes or so. (A good way to tell if it’s done is to scoop some up in a ladle and pour it back into the pot. You want it to fall in clumps and not be too runny). If you think it’s not there yet, let it cook for a little while longer.
Taste the jam; if you think it’s too tart, add more sugar and cook until it dissolves completely.
When ready, add the lime/lemon juice and ladle them into clean glass jars. Let them sit outside until they get to room temperature and then refrigerate.