While the potatoes are boiling, in another pot, add the onions, ginger, garlic and cashews. Add ½ cup water and let cook for about 5-8 mins on medium heat till the onions are soft. Place everything into a blender jar along with remaining liquid and make a fine paste. Keep aside.
In a small bowl, whisk yogurt with sugar, dried mint and garam masala. Keep near. Note- All the above steps can be done a day in advance. Lets begin cooking the sauce.In a heavy bottom cooking pot (wok shaped or kadai is best), warm up the ghee on low medium heat.
Temper with all the listed whole spices. Cook for 10-15 seconds.
Add the onion & nut paste that we made. On medium low heat, stirring, cook the paste for 5-7 mins or till you start see oil separating but the color of paste should not change much. Due to cashews the paste will stick so keep stirring regularly.
Next, add the coriander & red chili powder, saute for a minute. If needed add a little water while sautéing the spices.
Add the potatoes next and stir around such that all the potatoes are covered in the onion masala. Reduce the heat to low and keep stirring for 2-3 minutes.
Add the beaten yogurt to the pot next. Immediately start mixing and continue for 2-3 minutes so as to avoid yogurt from curdling.
Slowly you will see that yogurt forms part of the masala. Check and adjust the salt. We will continue to stir for few more minute till yogurt is warmed through. Slowly you will see fat from the yogurt separating in tiny bubbles.
Add ? cup to ¾ hot water next to the pot (depending on the sauce consistency you prefer). Mix around, let come to a slow boil and cover the pot. Let cook on low for 10 minutes on low heat till the potatoes are cooked through.
Serve garnished with chopped coriander leaves.