Put plums in a bowl, add the sugar, and juice of 1/2 a lemon. Mix well. Let it sit for about one hour. Check at 30 minutes, and mix once more.
Pour the plum, sugar and juices that have collected in the bottom of the bowl in a large non-reactive pot*. Place over medium-high heat. Stir and bring to a slight boil. Reduce heat to medium-low, and allow to simmer for about two hours. Stir every 15 minutes, to make sure not only the middle of your pot is cooking. It will become thick and turn into a beautiful dark color.
Allow the jam (compote) to cool in the pot and put into clean containers. It lasts in the refrigerator for about three to four weeks.