Position rack in center of oven and preheat the oven to 350°F.
In a medium bowl, mix together the graham cracker crumbs, 1/2 cup of the sugar, and the melted butter until the mixture resembles wet sand.
With clean hands, firmly press the mixture across the bottom and up the sides of a 9-inch pie pan.
Using oven mitts or pot holders, place pie pan in oven and bake until the crust is brown and firm, about 15 minutes.
Using oven mitts or pot holders, carefully remove the pie crust from the oven and let cool completely on a wire rack before filling.
In a separate bowl, whisk together the condensed milk, lemon juice, and egg yolks. Pour into the cooled pie shell.
Using oven mitts or pot holders, place pie in oven and bake until set, about 15 minutes. Remove from the oven using oven mitts or pot holders and transfer to a wire rack to cool. Turn off the oven.
While the pie is baking, make the meringue by placing the 6 egg whites in a large clean bowl and beating them with an electric mixer until soft peaks start to form.
Slowly add the remaining 3/4 cup of sugar while beating constantly. Beat until stiff, glossy peaks form. Be careful not to overbeat the meringue, as it will appear grainy or clumpy instead of smooth and glossy and will be difficult to spread.
Using a rubber spatula, spread the meringue evenly over the warm pie filling, smoothing out to the edges so the meringue won't pucker or get runny during baking.
With a dull knife, make decorative peaks in the meringue.
Using oven mitts or pot holders, position rack in the upper third of oven and preheat the broiler. If your broiler is separate from your oven, have an adult show you how to use it.
Using oven mitts or pot holders, place the pie under the broiler and cook until the meringue is golden brown, about 1 minute. Make sure to keep an eye on this it will brown quickly!
With oven mitts or pot holders, very carefully remove the pie from under the broiler. Refrigerate until thoroughly chilled, at least 2 hours.
Serve garnished with a sprig of mint and/or a lemon slice if desired.
YIELD: One 9-inch pie, serving 8 to 10