In a medium bowl, whisk together the sweetened condensed milk and freshly squeezed lemon juice until well combined. The mixture will thicken slightly as the acid from the lemon juice reacts with the milk—this is exactly what you want and creates the custard-like filling. Stir in the lemon zest and salt to distribute the flavors evenly throughout the mixture.

Gently fold the thawed whipped topping into the lemon mixture using a spatula, being careful not to deflate the whipped topping. Use a gentle folding motion rather than stirring to maintain the airy texture and create a light, mousse-like filling. I find that folding in three to four turns is usually all you need—over-mixing will make the filling dense and heavy.

Pour the filling into the graham cracker crust, smoothing the top with a spatula if desired. Transfer the pie to the refrigerator and chill for at least 4 hours, or until the filling is completely firm and set. The cold temperature will allow the filling to achieve the proper texture while the flavors meld together beautifully.