Mix the pears, cranberries, and lemon juice in a large bowl. Add 2/3 cup sugar and cornstarch and stir them into the fruit. Pour the filling into the pie shell.
Invert the top pastry over the filling and press the edges together. Trim the pastry flush with the edge of the pan and crimp with a fork. Poke several steam vents in the top. Lightly brush on the milk and sprinkle 1 teaspoon sugar.
Bake for 30 minutes. Reduce the oven temperature to 375° F. Bake until the juices bubble thickly, 25 to 30 minutes more. Cool 2 hours.