Preheat oven to 375 F. In a medium mixing bowl, toss together the plums, 1/2 cup sugar, almonds, mint, mint lemonade, cornstarch and cardamom. Pour into 10” signature skillet and set aside while making the biscuit topping.
In a large mixing bowl, whisk together the flour, 2 tablespoons sugar, baking powder, baking soda and salt. Cut in butter with a pastry blender or fingertips until mixture resembles coarse meal. Add the buttermilk and stir to combine into a soft dough. Dump out the dough onto a floured board, and gently knead 3 to 4 times. Roll dough out to 1/2-inch thickness and cut into round biscuits. Arrange biscuits on top of plums, spacing evenly but not quite touching.
Combine turbinado sugar and cinnamon in a small bowl. Brush tops of biscuits with the beaten egg, and sprinkle with the sugar mixture. Bake in preheated oven until fruit is hot and bubbling, and biscuits are golden brown about 35 minutes.