Prepare the pie crust and chill for 1 hour as directed (or up to 2 days). When ready to bake, place a rack in the center of your oven preheat to 425°F.
Divide the dough in two, making one half slightly larger than the other. Return the smaller half to the refrigerator.
On a moderately floured work surface (I like to use a piece of parchment or wax paper so I can easily move the dough), roll out the larger half into a 12-inch circle (if it’s too stiff from being in the fridge, let it rest at room temperature for a few minutes, until you can roll it). When rolling, always work from the center of the dough outward, rotating the dough every now and then as you go (this will give you a better, more even circle, less tearing, and you’ll be able to gauge if the dough is sticking to your work surface, which you do not want). Work fast, because the dough gets sticky as it warms. Flour the work surface and your rolling pin as little as is manageable (but don’t be afraid to use flour if dough is sticking). If at any point the dough gets too sticky to handle, just pop it back in the fridge to firm up for a few minutes (this is where the parchment paper comes in handy). If you have any tears or cracks along the way, don’t stress. Just patch them together.
Transfer the dough to a regular 9-inch pie plate (not deep dish)—I like to fold the circle over the rolling pin, then use the rolling pin to unfurl it gently into the plate. Ease the dough down into the plate (don’t pull to stretch it or it will bounce back). Pop into the freezer.
On a sheet of parchment or wax paper, roll out the second (smaller) portion of dough in an 11-inch circle; if you’d like to make a lattice, cut it into 1-inch strips. Use the paper to place it in the refrigerator while you prepare the filling.
In a large bowl, stir together the rhubarb, 3/4 cup granulated sugar, flour, salt, and nutmeg.
Remove the bottom crust from the freezer. Sprinkle the remaining 1 teaspoon granulated sugar over the bottom. With a large spoon, transfer all of the rhubarb mixture (including any juices if the rhubarb is fresh or dry bits of flour and sugar if it’s frozen) to the plate, spreading it into an even layer.
Scatter the butter over the top of the rhubarb filling.
Drape the top crust over the filling (or arrange the lattice). Trim the pie crust to 1/2-inch overhang all the way around. Tuck the top crust under the bottom crust. Crimp the edges with your fingers (or keep it easy and simply press along the edges with the tines of a fork). If not using a lattice, with a sharp knife, cut 5 to 6 vents in the top.
Use a dry brush to brush away any excess flour from the crust, then brush the crust all over with the eggwash.
Place the pie on a parchment-lined baking sheet and sprinkle with the turbinado sugar. Bake on the center rack at 425°F for 15 minutes, then reduce the temperature to 375°F. Continue baking until the crust is deep golden and you see bits of filling bubbling out of the vents, about 50 to 55 minutes more, rotating the pan 180° halfway through (if your rhubarb was frozen, you might need to extend the baking time by 10 minutes or more). If at any point, the crust edges get too dark, shield the edges with foil or a pie crust guard.
Let the pie cool at room temperature COMPLETELY! (This will take at least 4 hours and is hard but worth it). Slice and enjoy with whipped cream, or nothing at all!