Melt the butter and sugar together with the lime juice and zest. Tip in the rhubarb and cook for three minutes, until softened.
Add the chopped mint and set aside to cool.
Spoon a scoop of the ice cream into the base of a tall glass.
Sprinkle over a third of the sultanas and nuts, and a third of the cooled rhubarb.
Repeat the layering until all ingredients are used up.
Garnish the sundae with a mint sprig and serve straightaway.