For the buttermilk shortcakes: Preheat the oven to 425 degrees F.
Chill 2 large mixing bowls, a pastry cutter, the flour and cubed butter in the freezer for 15 minutes before assembling the shortcakes.
Add the flour to one of the chilled mixing bowls. Working quickly, cut the butter into the flour using the chilled pastry cutter (or your fingers); the goal here is pea-size balls of butter. In batches, add the buttermilk until the dough is cohesive yet still moist. (You may not need all of it.) Turn the dough out onto a floured surface and lightly press into a 1-inch-thick round. Using a 4-inch biscuit cutter or the business end of a dry measuring cup, cut out 8 biscuits.
Preheat a cast iron skillet in the oven for 5 minutes.
Swirl the oil in the bottom of the skillet and gently arrange the biscuits in the skillet, making sure they are all nestled up against each other tightly.
Brush the tops with the melted butter and sprinkle with the sanding sugar. Bake until the tops are just slightly golden brown, about 15 minutes.
For the plums and cherries: Heat a 12-inch nonstick skillet over medium heat. Add the plums, cherries, granulated sugar, lemon zest and juice, ginger and cinnamon. Cook, stirring very gently as the fruit releases its juice, until the fruit is slightly soft, a few minutes. Take off the heat and add the butter. Gently stir until melted.
For the ginger cardamom whipped cream: In the other cold bowl, mix together the cream, granulated sugar, vanilla, cardamom and ginger. Beat to medium peaks.
For assembly: Split a warm shortcake with a fork. Place the bottom on a plate or shallow bowl and spoon on some of the warm plum and cherry mixture. Dollop on some whipped cream and sprinkle with some candied walnuts. Top with the top of the shortcake. Spoon on some more of the warm plum and cherry mixture, dollop on more whipped cream and sprinkle with more candied walnuts. Repeat with remaining ingredients to make 7 more.