Cover and chill the egg whites. In a medium bowl, combine the egg yolks, lemon juice, and condensed milk. Mix well and pour over the graham cracker crust.
Bake until stiff, about 30 minutes. Let cool to room temperature, then chill for at least 3 hours or up to overnight.
When ready to assemble the pie, preheat the oven to 350°F.
In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites and cream of tartar on high speed. With the mixer running, gradually add the sugar and beat until stiff peaks form.
Spread the meringue over the pie filling.
Bake until the meringue is golden brown, about 12 minutes. Let cool for 30 minutes, then chill for 30 minutes before serving.