Step 1Put 3 metal spoons in the freezer. Combine the plums, lemon juice, salt and 1 tablespoon water in a medium saucepan. Bring to a simmer over medium heat, stirring occasionally. Reduce the heat to medium-low and simmer, stirring occasionally, until the fruit starts to soften a little, about 3 minutes.
Step 2Stir in the sugar. Bring the mixture to a boil over medium-high heat, stirring occasionally. Reduce the heat to maintain a steady low boil. Continue boiling, stirring and skimming off foam from the surface occasionally, until the mixture is thickened and drizzly like maple syrup, 12 to 15 minutes. To test the jam’s doneness, take a spoon from the freezer and scoop a little onto the tip. Tilt the spoon; the jam should glide as slow as honey. Repeat the testing with the remaining spoons if needed.
Step 3Divide the jam among three clean half-pint canning jars. Seal and cool to room temperature, then refrigerate.
Star Anise Plum Jam: Add 3 whole star anise to the saucepan with the plums in Step 1. You can jar the jam with the star anise, but discard before eating.
Spicy Plum Jam: Add 1 fresh seeded and minced habanero or Scotch bonnet chile pepper to the saucepan with the plums in Step 1.