
Preheat the oven to 425°F with the oven rack in the lower third of the oven. (If using a homemade crust, parbake it first. See my notes for the instructions!)
In a large bowl, whisk together the flour, brown sugar, cinnamon, salt, and nutmeg. Using a pastry cutter or fork, cut in the cold butter until the pieces are no larger than the size of peas. Set the crumb mixture aside.
In a large bowl, whisk together the molasses, water, and baking soda until well combined. Sprinkle half of the crumb mixture (about 1¼ cups) evenly in the pie crust. Pour the molasses mixture into the crust over the crumb mixture, and gently sprinkle the remaining crumb mixture evenly over top.
Bake the pie for 15 minutes.
Reduce the oven temperature to 350°F and continue baking until the center of the pie is set and barely wobbles when gently shaken, 20 to 25 minutes. Transfer the pie to a wire rack and cool completely, about 2 hours before slicing.