In a large bowl, toss the plums, sugar, and lemon juice. Cover and refrigerate for at least 8 hours.
When ready to cook, put a small plate in the freezer.
Transfer the mixture to a heavy medium pot and bring to a boil over high heat. Turn down the heat to maintain a gentle simmer. Cook, stirring and scraping the pot as needed, until the juices are concentrated and syrupy but not caramelized, 15–18 minutes. (Use a candy thermometer to ensure the mixture never exceeds 220°F.) Skim any foam that forms at the surface.
Retrieve the plate and drizzle a small drop of the plum mixture on the surface. If it starts to set and form a skin after a minute, the preserves are ready. If it remains runny, return the plate to the freezer and cook the plum mixture a few minutes more before testing again. Stir in the herbs (if using), bring to a boil, then immediately remove from the heat.
Follow recommended water-bath-canning procedures to jar and preserve the jam for long-term storage. Or, let it cool completely, then refrigerate and serve immediately or within the next 2 weeks.