Place condensed milk and lime zest in a saucepan over medium heat until the milk starts to simmer. Turn down to a low heat and simmer for a few minutes until the condensed milk starts to thicken slightly.
Remove from heat, add the white chocolate and mix until the chocolate has melted.
Mix in the nuts and cranberries and pour into the prepared tin.
Refrigerate for at least 2 hours before cutting into squares of desired size.