1Preheat the oven to 375°F. In a medium bowl, stir together the graham crumbs, brown sugar and butter; mix with a fork until well blended, then press the mixture into the bottom and up the sides of a 9-inch pie plate. Bake for 12-15 minutes, until pale golden around the edges. Increase the oven temperature to 400°F.
2In a medium saucepan, whisk together 1 3/4 cups of the sugar and the cornstarch. Whisk in the water, lemon juice and zest, then the egg yolks. Set over medium-high heat and cook, whisking almost constantly, until the mixture boils and thickens. Pour into the crust.
3If you're using it, stir the cream of tartar into the remaining 1/2 cup sugar. Using an electric mixer, beat the egg whites until soft peaks form. Gradually add the sugar, beating constantly until glossy and stiff. Mound on the warm filling, spreading it to the edges to prevent it from shrinking. Bake for 5-10 minutes, or until the meringue is golden.
4Let cool on a wire rack for a few hours, until set.