In a large mixing bowl, whisk together the milk, lemon pudding mix, and lemonade concentrate until thickened, about 2-3 minutes. (Whisk immediately once the lemonade is added to the milk to prevent curdling.)
Fold in the Cool Whip and lemon zest with a rubber spatula until evenly combined.
Pour the lemon filling into the pie crust and spread into an even layer.
Chill for at least 8 hours or until set.
Remove from the fridge and add swirls of whipped cream around the edge of the pie. Cut into 8 slices and garnish with half lemon slices if desired.
Notes
Cover and store any leftovers in the fridge for up to 3 days. Freeze the whole pie or slices for up to 2 months. Thaw in the fridge overnight or serve frozen.