Mix all ingredients in a large microwave safe bowl and blend in a blender at low speed.
Transfer back to the bowl, and heat in the microwave for about 8-10 mins until the mix comes to a gentle boil. Watch while heating to ensure it doesnt foam and boil over.
Remove and mix well with a ladle. Blending for about 30 seconds helps too.
Allow to cool, and transfer to a non-reactive 13X9 cake pan (i.e made of glass or ceramic).
Cover and freeze for atleast 6 hours. Cut to cubes and serve with a few crushed pistachios for garnish.