Wash the plums and then cut in half, remove the pit and dice.
Transfer to a large pot or pan, spread evenly and start browning them on a medium high heat. No need for any sugar or sweeteners. Plums are sweet and they will get even sweeter when they release juices.
Stir, cover and lower the heat to low and cook for 10 minutes, until the plums start releasing their juices.
Cook uncovered, stirring occasionally until all the juice reduces.
Prepare a medium size pan (8 inch by 8 inch) and transfer the plums into it, be careful its very hot.
For crumble combine 2 tablespoons of room temperature butter, 1/3 cup all purpose flour, 1/3 cup of old fashioned oats, and 2 tablespoons of brown sugar.
Spread crumble over the fruit.
Bake it at 375F for 35-45 minutes, until the crumble on top browns.
