Preheat the oven to 180°C (160°C fan forced). Grease a 33cm x 22cm rectangular cake pan and line the base and sides with baking paper.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Place the cake mix, eggs, butter and milk in a bowl, then whisk (by hand or using an electric mixer) for 2-3 minutes until smooth and creamy. Whisk in the lemon zest. Pour into the prepared pan and gently level the surface. Bake for 25-27 minutes until risen and golden and a skewer inserted into the cake comes out clean. Remove from the oven and cool in the pan for 10 minutes.
With the handle of a wooden spoon, poke holes at 1-2cm intervals almost to the bottom of the cake, wiping the spoon handle occasionally to reduce sticking.
To make the topping, stir the condensed milk, cream, lime juice and zest together in a bowl – the mixture will gradually thicken. Leave the cake in the pan, then spread two-thirds of the mixture all over the cake and use the back of a spoon to spread evenly over the surface, working back and forth to fill the holes. Spread the rest of the mixture over the top of the cake and swirl with a palette knife. Chill in the fridge for 2 hours.
Carefully remove the cake from the pan using the baking paper. Slice into 15 servings and decorate each with lemon and lime slices, sliced strawberries and mint leaves. Refrigerate any leftovers in an airtight container for 2-3 days.