Prepare the Jell-O base: Dissolve the lemon Jell-O in 2 cups of boiling water in a large mixing bowl. Stir continuously until the powder is completely dissolved with no granules remaining. This creates the foundation for your dessert's structure and lemon flavor.

Thin and chill the mixture: Add 1 cup of cold water to the dissolved Jell-O and mix well to combine. Place the bowl in the refrigerator until the mixture begins to thicken but isn't completely set, approximately 30-40 minutes. The mixture should have the consistency of egg whites - slightly jiggly but not liquid.

Prepare the lemon filling: While the Jell-O is chilling, prepare the lemon pie filling according to the package directions. This typically involves cooking it on the stovetop until thickened. Once cooked, allow it to cool completely, placing plastic wrap directly on the surface to prevent a skin from forming.

Combine Jell-O and lemon filling: Once both components are ready, gently fold the cooled lemon pie filling into the thickened Jell-O. Use a spatula and fold carefully until the mixture is well blended and smooth, with no streaks of pie filling visible.

Add the Cool Whip: Fold in the thawed Cool Whip using gentle, sweeping motions to maintain as much air as possible in the mixture. This creates the signature fluffy texture that makes this dessert special.

Chill until set: Transfer the finished mixture into a large serving bowl or portion into individual serving cups. Return to the refrigerator and chill for at least 1 hour, or until the dessert is completely set.

Garnish and serve: Just before serving, top with optional whipped cream and fresh lemon slices for an elegant presentation and extra burst of flavor.