Choose a round springform pan and cover the base completely with ladyfingers, trimming if necessary. In a saucepan, combine 1 cup of water with 2 tablespoons of sugar and 1 tablespoon of lemon juice. Stir continuously without boiling for 4 minutes, then let it cool until warm.

Pour the cooled water mixture over the ladyfingers to soak them well. To make the lemon mousse, heat the milk in a saucepan until just before boiling, then add the gelatin and stir until completely dissolved. Add the lemon juice and sweetened condensed milk, remove from heat, and blend everything together. Let it cool until warm.

Whip the heavy cream, which should be very cold (I take it out of the fridge and put it in the freezer for 10 minutes), until it has a minimum of 35% fat content. Whip the cream with a whisk, but it doesn't need to be fully whipped as it will be mixed with the lemon mousse (it can be semi-whipped). Add the cream to the warm lemon mousse mixture and fold gently until well combined. Pour into the pan and freeze for 30 minutes, then refrigerate for 1.5 hours.

After the cake has set for 2 hours and is firm, prepare the gin tonic topping. Soak the 8 sheets of gelatin in room temperature water for a few minutes. In a saucepan, combine the water and sugar, stirring until dissolved. Add the drained gelatin, remove from heat, and blend everything together. Then add the lemon juice, gin, and tonic water, stirring well, and let it cool until warm.

Once the topping mixture is warm, strain it and pour over the set cake. Freeze for 30 minutes, then refrigerate for a few more hours until fully set. After a few hours, ensure it is set, then carefully remove from the pan using a knife around the edge to prevent sticking. Garnish with lime slices around the cake and a bit of grated lime zest on top (if you plan to consume it later, add the lime just before serving).

If you don't like alcohol, you can follow all the steps and replace the gin with more lemon juice (taste it first to adjust to your desired strength).