Have ready and refrigerated one crimped 9-inch pie shell and oat crumble topping.
Juice the lemon into a large bowl, removing any seeds. Core, peel, and slice the pears with a sharp knife. Dredge the pear slices in the lemon juice. Sprinkle with the granulated sugar. Set aside to soften slightly and release some of the juices, 20-30 minutes.
Meanwhile, position the oven racks in the bottom and center positions, place a rimmed baking sheet on the bottom rack.
Preheat the oven to 425F.
In a small bowl, stir together the brown sugar, flour, ginger, allspice, white pepper, and salt. Drain any excess juices from the pears. Toss the pears with the sugar mixture. Sprinkle with the bourbon, bitters, and vanilla. Tightly layer the pears in the prepared pie shell so that there are minimal gaps, mounding them slightly higher in the center.
Top the pie with the oat crumble.
Place the pie on the rimmed baking sheet on the lowest rack of the oven.
Bake at 425F for 20-25 minutes, or until the pastry is set and beginning to brown. Lower the heat to 375F, move the pie to the center oven rack, and continue to bake until the pastry is a deep golden brown and the juices are bubbling, about 30-35 minutes longer. Test the pears with a skewer or a sharp knife. They should be tender.
Allow to cool to room temperature on a wire rack, 2-3 hours. Serve slightly warm or at room temperature.
The pie will keep refrigerated for 3 days or at room temperature for 2 days.