Preheat your oven to 375℉ and get a 9x7" baking dish.
In a medium bowl, toss together sliced plums, lemon zest, corn starch, salt, and sugar. Transfer to the baking dish / casserole dish.
In a large bowl, mix together the flour, remaining sugar, baking powder, salt, and cardamom.
Cube your cold butter. Work it into the dough with your hands in a pinch and mix motion until you have some small pieces (pebble-sized) and a few larger, incorporated with the dough.
Pour the milk into the flour mixture and use a wooden spoon to mix together until a wet dough forms.
Spoon dollops of dough on top of the plums in the baking dish. Then sprinkle it with raw sugar.
Bake 40-45 minutes until the fruit is bubbling around the edges and the biscuit topping is golden brown.