Place sugar in a saucepan over medium heat and let caramelise, without stirring. When it's evenly melted and golden brown, add the boiling water and mix well. Transfer the caramel to a 23cm tube cake tin or silicone mould.
Whisk together the condensed milk, milk, eggs and lime zest. The longer it's whisked, the lighter the end result will be. Pour the mixture into the tin over the caramel, cover with foil and place in a water bath.
Transfer to the oven and bake for 30 to 40 minutes, or until set.
Let cool slightly, then turn out onto a plate and chill in the refrigerator for at least 3 hours.