Place plum slices and sugar in a large bowl and mix to coat thoroughly. Let sit for 30 minutes to one hour, or up to a full day for a shorter cooking time. Place a small plate in the fridge—you'll use this to test your jam for doneness later on.
After the plums are macerated, transfer the mixture to a large non-reactive pot, add the lemon zest and juice and the sage bundle, and turn heat to medium. Bring the mixture to a boil, stirring occasionally to make sure the bottom doesn't burn.
Reduce heat to medium-low and continue to cook until the jam starts to thicken, 25-30 minutes. Stir occasionally, scraping the bottom and sides. After 10 minutes, taste for sage infusion—if it's strong enough, squeeze the sage bundle against the side of the pot and discard. If you want more, leave it in another 5-10 minutes, then taste test again.
While the jam cooks, wash and dry your jars and lids, and, if canning, prepare your boiling water bath.
Once you feel jam is starting to set, pour a small spoonful onto the chilled plate from the freezer. Return to the freezer for 2 minutes. Remove, and use your finger to draw a line through the jam. If the lines remain clear, and it's not too runny, it's likely done.
Once done, funnel the jam into clean jars, seal, and process according to your canning process. If not canning, let cool, then store in the fridge for up to a month. Serve with toasts, yogurts, cheeses, and other treats.