Line the inside bottom of a 9-inch springform pan with aluminum foil or parchment paper. Set aside.

Process the Oreos in a food processor until ground into fine crumbs.

Stir in the melted butter until the cookie crumbs are well coated.

Press the crumbs tightly into the bottom of the prepared pan and freeze while you make the filling.

Sprinkle the gelatin over the 3 tablespoons of cold water in a small microwave safe bowl and set aside.

In a medium bowl, combine the heavy cream, Mascarpone cheese, confectioners' sugar, vanilla, and salt. Beat on medium low speed until whipped and fluffy; about 5 minutes.

Add the strawberries and lemon juice to the bowl of a food processor and process until smooth. Run the puree through a fine mesh sieve and discard the seeds.

Add ¼ cup of the strawberry puree to the gelatin. Heat in the microwave for 1 minute then stir until the gelatin is fully dissolved. Pour back into the bowl with the remaining puree and stir to combine.

Pour the strawberry mixture into the bowl with the whipped cream mixture. Use a silicone spatula to carefully fold everything together until well combined.

Pour the strawberry mousse over the prepared crust. Use an offset spatula to smooth the top.

Cover with plastic wrap and refrigerate for 2 hours.

Sprinkle the gelatin over 2 tablespoons of cold water in a small microwave safe bowl and set aside.

Puree the strawberries and sugar in a food processor until smooth. Run the puree through a fine mesh sieve and discard the seeds.

Add ¼ cup of the puree to the gelatin and heat in the microwave for 1 minute. Stir until the gelatin is fully dissolved. Pour back into the bowl with the remaining puree and stir to combine.

Pour the puree over the top of the mousse layer and swirl the pan slightly until the puree evenly covers the top.

Cover the pan with plastic wrap and refrigerate for at least 6 hours or overnight.

Just before serving, pipe swirls of whipped cream over the top if desired.

Run a lightly oiled knife around the outer edge of the cake to loosen it from the pan. Release the clasp and remove the outer ring of the pan. Transfer the mousse cake to a serving platter.