Meanwhile, prepare the lemon pie filling. In the bowl of a standing mixer, add the egg yolks. Whisk until they begin to pale and are thickened, about 5 minutes.
Add the lemon zest and sweetened condensed milk, mixing until completely combined, about 2 minutes.
Stir in the lemon juice. Pour mixture over parbaked graham cracker crust and return to oven for another 10 minutes, rotating the pan halfway through cooking.
Allow pan to cool before placing in the refrigerator to completely chill for at least 4 hours. This allows the filling to set, which is key!
For the whipped cream:
Place chilled cream into a metal or glass bowl (bonus if you can chill the bowl in advance - this will help your cream whip faster!). Whip until soft peaks form. Add powdered sugar continuing to whip until stiff peaks form.
Top lemon pie with whipped, garnishing with fresh slices of lemon.