Boil the milk in a large non-stick kadai on high heat. Add the alum and sugar, and continue to cook, stirring continuously, for about one and a half hours, or till the milk becomes grainy and begins to thicken.
Add the ghee and glucose; mix well and continue to cook till the mixture begins to coat the back of the ladle.
Transfer the mixture into a greased ten-by-eight-inch aluminium tray , sprinkle the pistachios and set aside to cool and set.
Cut into squares and serve.