Macerate the blackberries with the tarragon.
Mix the dry ingredients for the biscuit topping.
Cut in the cold butter until mixture is crumbly.
Stir in the heavy cream to form a biscuit dough.
Spread the macerated blackberries in a baking dish.
Dollop the biscuit dough on top of the blackberries.
Sprinkle the turbinado sugar over the top.
Bake until the topping is golden brown and the filling is bubbly.
