Pre-heat the oven to 400°F and arrange a shelf slightly below the center of the oven. Butter the bottom and sides of a 9- or 10-inch cast iron skillet.

In a large mixing bowl, stir together the flour and sugar. Grate the cold butter into the flour and stir it in to coat evenly. Stir in the lemon zest and the buttermilk. When the dough comes together gently fold in the blueberries. The dough will be quite wet.

Spread the dough into the prepared skillet. Sprinkle the turbinado sugar over top the batter.

Place the skillet on the arranged shelf in the oven. Bake until lightly browned - about 25 minutes. Brush the tops of the biscuits with some melted butter and pop them back into the oven for another 5 minutes or so, until nicely browned.

Turn the biscuit out onto a plate, and then invert it again so it is right side up. Cut into wedges and serve warm with whipped cream or top with a little powdered sugar.