For the filling: In a large bowl, combine the granulated sugar and cornstarch; whisk until thoroughly mixed. Add the berries and lemon zest, and stir until the berries are completely coated. Pour the mixture into a 2-quart casserole dish or into 6 individual ramekins.
For the topping: Sift together the flour, granulated sugar, baking powder and salt in a large bowl. Cut the butter into the dry ingredients with a pastry blender or two knives until the mixture has the texture of coarse meal.
Add the cream and mix in with a fork until a dough forms. Dust a work surface and your hands with flour. Gather the dough into a ball.
Knead the dough gently for about 1 minute. Flatten the dough ball and then roll out with a lightly-floured rolling pin, shaping the dough into a rectangle or oval that matches the top of your casserole dish; cut into rounds if using ramekins.
Lay the dough on top of the berries in the casserole dish or ramekins. In a small bowl, whisk the egg with 1 tablespoon water until milky. Brush the dough with the egg wash and sprinkle with the turbinado sugar.
Bake the cobbler until the topping is golden brown, about 35 minutes. Let rest for at least 10 minutes before serving. Serve with ice cream if using.