In a pan heat half the oil and fry cashews till golden. Remove and keep aside.
To the same oil add semolina and roast on low till pinkish brown. Keep aside.
Heat the remaining oil for tempering and add mustard seeds.
After they shutter add remaining tempering ingredients and roast for a minute.
Pour over the semolina and mix well.
Add coriander leaves.
At this point you can place the Idli steamer to heat .
Stir in the yogurt, and let stand for a minute or two .
Add the buttermilk.
Add the fried cashews, cooking soda and salt.
Smear the Idli plates with oil.
Pour sufficient batter into the cavities and steam for about 10 minutes.
Cool for a couple of minutes before scooping out the idlis.
Serve right away with chutney.