Preheat the oven to 425℉. Line a 9-inch pie plate with the pie crust, and crimp the edges.
PREPARE THE PIE:
In a large saucepan, whisk together the water, corn syrup, lemon juice and lemon zest. Whisk in the cornstarch and ginger. Gently stir in the pears. Bring to a boil over medium-heat; cook and stir for 1 minute or until thickened. Remove from the heat; add the butter. Pour the pear filling into the pastry shell.
ADD THE TOPPING and BAKE:
In a small bowl, combine the flour, brown sugar and ginger. Use a pastry cutter (or two knives) to cut in the butter until crumbly. Stir in the pecans. Sprinkle the topping evenly over the filling. Bake for 20 to 25 minutes (or until the filling is bubbly and the topping is browned. Cool on a wire rack. Serve with a scoop of vanilla ice cream, if desired.