For the oatmeal crumb: Use a food processor to grind ¾ cup of the oats, pulsing the machine until the oats resemble coarse cornmeal. Add the flour, sugar, ginger, nutmeg, and salt and pulse 5 times to combine. Transfer the mixture to a bowl and whisk in the remaining ¼ cup whole oats.
Add the melted butter and blend with a fork or your fingers until the butter is incorporated and the mixture gathers into small clumps. Set the bowl in the refrigerator and chill the crumb for 15 minutes before topping your pie.
For the pie: Lightly flour a smooth work surface and a rolling pin.
Take a chilled disk of dough out of the fridge. Give it a couple of firm squeezes just to say hello, then unwrap it and set it on the floured work surface.
Set the pin crosswise on the dough and press down firmly, making a nice deep channel across the full width of the disk. Turn the disk 180 degrees and repeat, making a second indentation, forming a plus sign.
Use your rolling pin to press down each of the wedges, turning the dough 45 degrees each time. This will give you the beginnings of a thick circle.
Now, rolling from the center outward and rotating the dough a quarter turn to maintain a circular shape, roll the dough out to a 13-inch circle with an even thickness of ¼ inch.
Set your 9-inch (23-cm) pie pan alongside the circle of dough. Brush off any loose flour, carefully fold the dough circle in half, transfer it to the pan, and unfold.
At this point, the dough will be lying across rather than fitted into the pan. Now, without stretching the dough, set the dough down into the pan so that it is flush up against the sides and bottom. The best way to do this is to gingerly lift the dough and gently shift it around so that it settles into the pan bit by bit. Use a very light touch to help cozy it in.
To flute the edge, fold the overhang under to form a 1-inch wall that rests on the lip of the pan with the seam slightly below the pan’s top edge. Go around the edge of the pan and use a very light touch to firm up the wall to an even thickness from the bottom to the top and all the way around. Flute the edge of the crust at about 1-inch intervals, pressing from the inside with the knuckle of your index finger while supporting on the outside with the thumb and index finger of your opposite hand. Don’t pinch the dough, you want the flute to look like a thick rope.
Transfer the crust to the refrigerator to chill while you make your filling. Alternatively, at this point the crust can be covered tightly in plastic wrap and refrigerated for up to 3 days or double-wrapped and frozen for up to 2 months (defrost overnight in the refrigerator before filling and baking or prebaking, or at room temperature for 30 minutes).
Preheat the oven to 375°F with a rack in the center. Line a rimmed baking sheet with parchment paper or foil. (I actually pre-heated my baking sheet in the oven until ready to bake the pie).
In a large bowl, toss the pears with the lemon juice and vanilla.
In a small bowl, whisk together the sugar, cornstarch, candied ginger, and salt. Sprinkle the sugar mixture over the pears and toss to coat the fruit and moisten the sugar and cornstarch so that no dry white streaks remain.
Retrieve the prepared pie shell from the refrigerator and set the pan on the parchment-lined baking sheet. Scoop the filling into the shell and top with the oatmeal crumb, spreading evenly and completely covering the fruit.
Transfer the baking sheet to the oven and bake the pie 25 minutes, then rotate the baking sheet, lower the oven temperature to 350°F, and bake 25 to 30 minutes more, or until the topping is golden and the juices bubble up through the crumb (all of my pies bake longer than recipes suggest–mine baked an extra 25 minutes beyond, a total of 55 minutes at the lower temperature–but just look for the signs and don’t count on the timer to always be right). Tent the top with foil if it starts to over-brown.
Set the baking sheet on a wire rack and let the pie cool and set, uncovered, at room temperature, overnight (or up to 3 days) before slicing and serving (mine cooled about 4 to 6 hours, which was plenty of time for it to set while retaining its flaky crust; I also dusted mine with confectioners’ sugar before serving).