For the meringue topping:
1 cup granulated sugar
2 tablespoons all-purpose flour
3 tablespoons cornstarch
¼ teaspoon salt
1 ½ cups water
Juice and zest of 2 lemons
2 tablespoons unsalted butter
4 egg yolks, beaten
1 (9-inch) pie crust, baked
4 egg whites
½ cup granulated sugar