Set the frozen berries in a colander over a bowl, and let them thaw until most of the liquid has drained away. This usually takes about 30 minutes to 1 hour. Pat the berries dry with paper towels before making the filling.

While the berries thaw, prepare the crust. Add the flour, sugar, and salt to a mixing bowl, and whisk to combine. Work in the cold butter with a pastry cutter, two forks, or your fingertips until the mixture looks crumbly.

Shape the dough into a disc, wrap it in plastic wrap, and refrigerate it for at least 30 minutes.

Preheat the oven to 375°F, and grease an 8x8-inch baking dish with butter or nonstick cooking spray.

Place the thawed, drained berries in a large bowl. Add the sugar, cornstarch, cinnamon, nutmeg, and salt, then toss gently until the berries are coated. Spoon the filling into the prepared baking dish.

Lightly flour the counter, then roll the chilled dough from the center outward. Turn the dough as you roll until it is about 9 to 10 inches wide.

Whisk the egg and water together, if using, and brush it over the crust. Cut a few small slits in the dough so steam can escape.

Set the baking dish on a foil-lined baking sheet, and bake for 40 to 45 minutes or until the crust is golden and the filling bubbles around the edges and through the slits.

Let the cobbler cool for 15 minutes before serving. Serve with vanilla ice cream, if desired.