Preheat your oven to 375°F.
Combine 2/3 cup oat flour with 1 egg white in a bowl. Mix until a dough forms. Press the mixture evenly into a small, pie-shaped pan to form a thin crust.
Bake the crust in the preheated oven for about 8-10 minutes until lightly golden. Remove and set aside.
In a medium saucepan over medium heat, whisk together 1 cup fat-free Greek yogurt, 2 egg whites, the juice and zest from 2 lemons, and 1 tbsp cornstarch. Continue whisking as the mixture warms, allowing it to thicken into a smooth custard. Once thickened (about 5-7 minutes), remove from heat.
Pour the warm lemon custard into the pre-baked oat crust, smoothing the top.
In a clean bowl, whip 2 egg whites until soft peaks form. Gradually add 2 tbsp granulated sugar while continuing to whip until stiff, glossy peaks form.
Spread the meringue evenly over the lemon custard, ensuring the edges touch the crust to seal in the filling.
Place the pie back into the oven and bake for an additional 8-10 minutes, or until the meringue is lightly browned and crisp on top.
Allow the pie to cool slightly before serving to let the custard set further. Enjoy your Fresh Lemon Custard Pie either at breakfast, lunch, or dinner.