In a medium saucepan over medium-high heat, bring 1/2 cup water to a boil. Add in the jello and sugar, stir until dissolved completely.
Pour the jello mixture into a shallow dish (so it spreads out and cools faster). Refrigerate the jello for 20 to 25 minutes until it begins to cool and thicken to a syrup-like consistency.
Place the Cool Whip into a large mixing bowl. Pour in the cooled jello and mix to combine.
Pour in the strawberries and marshmallows. Fold them into the Cool Whip.
Pour the pie filling into your prepared crust. Use a spoon or rubber spatula to smooth the filling into an even layer. (When I used a store-bought crust I had more filling than the pie shell could hold. We put the rest of the filling in a Tupperware and popped it in the fridge for dessert another night.)
Cover the pie with plastic wrap and place in the fridge for at least 2 hours to set the filling, or until ready to serve. (If your filling isn't setting as quickly as you need, you can pop the pie in the freezer for 30 minutes to 1 hour.)
When you're ready to serve the pie, remove the plastic wrap and decorate the edges of the pie with whipped cream. I like to put a dollop of whipped cream in the center and put a whole strawberry on top.
If you don't want to use Cool Whip for this recipe you could substitute a stabilized whipped cream instead.