Originally found here: http://glutenfreecooking.about.com/od/glutenfreepiesandtarts/r/gflemonpie.htm
Preheat oven to 325° F / 163° C
1. Combine water, cornstarch, salt and sugar in a medium saucepan. Whisk until blended. Cook over medium-high heat, whisking constantly until thick, about 3 minutes.
2. Remove from heat and stir in lemon juice, lemon peel and butter. Whisk until combined.
3. Slowly drizzle egg yolks into the mixture while whisking. Return to medium-high heat and cook another 2-3 minutes. Cool pie filling mixture.
4. Use an electric mixer or stand mixer to beat egg whites with salt until foamy. Gradually add sugar and beat on high until stiff peaks form.
5. When pie filling has cooled to about room temperature, stir 2 tablespoons of meringue into the pie filling.
6. Pour the filling into the cool, pre-baked pie crust.
7. Use a spatula to pile the remaining meringue on the pie. Spread the meringue evenly over the filling, making sure to touch the edges of the crusts with meringue to prevent it from shrinking. Use a teaspoon or small knife to make swirls in the meringue.
8. Bake in preheated oven for about 15 minutes, or until the meringue peaks are lightly browned.