Preheat the oven to 400°F.
If you're not using a premade crust, roll your dough using a rolling pin, then carefully drape it into a 9-inch pie pan. Press the crust into the pan, trim the excess dough, and crimp the edges. Use a knife to poke holes in the bottom and sides of the pie crust.
Line the inside of your pie crust with foil or parchment paper, then fill it with pie weights, dried rice, or beans. Bake the crust for 15 minutes. Place it on a cooling rack and carefully remove the foil and weights when it comes out of the oven. Leave the oven on.
While the crust bakes, make the topping. First, stir the flour, brown sugar, cinnamon, nutmeg, and salt together in a large bowl.
Cut the butter into about eight slices, then add them to the flour mixture. Use a pastry cutter to cut the butter in until the mixture resembles fine crumbs. You can work it in a bit with your hands as well, but work quickly to avoid warming up the butter too much. Place the bowl in the fridge to chill while you make the filling.
To make the filling, stir the water and cornstarch together in a small saucepan. Place the saucepan over high heat and bring the liquid to a boil. It should thicken slightly. Remove the pot from heat.
Whisk the molasses and baking soda into the cornstarch mixture. The baking soda may cause it to bubble a bit.
Remove the crumble topping from the fridge. Pour half of the molasses mixture into the pie crust, then sprinkle half of the crumble topping over it.
Repeat the layers, spreading the remaining molasses filling over the crumble mixture, then arrange the remaining crumble mixture over the top layer of filling, spreading it out as evenly as you can and very gently pressing the crumble into the filling.
Place the pie into the oven. You may want to place something under it to catch drips. Bake the pie for 15 minutes at 400°F, then turn the temperature down to 350°F and bake the pie for 25 to 30 minutes more, until the filling is bubbly and the topping is golden brown. Keep an eye on the pie and cover the edges with foil or a pie shield as it darkens to prevent burning.
Remove the pie from the oven and place it directly onto a cooling rack. Let the pie cool completely, which should take at least three hours, before slicing and serving.