Line the pan with parchment or storage wrap before adding the mixture so that the cake can be removed once it is set. Once the cake is fully set, lift it out of the pan using the parchment or storage wrap overhang.
In top of a double boiler or a metal bowl over simmering water, combine both applesauce and gelatin until gelatin is dissolved, about 5 minutes.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Whip applesauce mixture on high speed until pale and creamy, 8-10 minutes. Spread into a greased 11x7-in. baking dish. Refrigerate until set, about 2 hours. Top with whipped topping and if desired, sliced strawberries, before serving.