I’d probably start by putting the plums in a pot with just enough water to keep them from scorching. Bring it to a simmer and then cook on medium low until the plums are all burst (15-20 min). Then, run the plums through your food mill and weigh the pulp. Calculate 40% of the weight and add that much sugar. Taste for acidity, add more sugar if you think it needs it, then proceed as normal for making jam. I like to cook it hot and fast to get rid of excess water before the flavor dulls, but others swear by low and slow.
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