Step 1

Heat 2 cups of water to boiling by microwaving in a large glass bowl for 4-5 minutes. Stir in contents of the Jell-O package until it dissolves. Separate the pineapple juice from the crushed pineapple by draining the juice into a small saucepan and setting aside for later.

Step 2

Add marshmallows to the hot lemon Jell-O and stir until dissolved, then stir in the crushed pineapple. Stir in 2 additional cups of cold water, and pour into a large rectangular glass dish. Transfer the Jell-O to the fridge and allow to set completely, about 3-4 hours.

Step 3

While the Jell-O is setting up in the fridge, prepare the pineapple whipped cream topping by whisking the beaten egg, flour, and 1/4 cup sugar with the reserved pineapple juice in a small saucepan and heating over medium-low heat, whisking constantly, until the sauce begins to thicken, about 4 to 6 minutes. Remove from heat and cool completely.

Step 4

In another bowl, beat the heavy cream with an electric mixer on high speed, gradually adding the additional 2-3 tablespoons of sugar and vanilla, until whipped cream forms and holds its shape when beaters are removed.

Step 5

Fold the cooled pineapple juice mixture into the whipped cream until combined, then spread over the completely set Jell-O. Sprinkle with the chopped walnuts and refrigerate until ready to serve.