Heat oil in a pressure cooker. Add mustard seeds and cumin seeds.
Once the mustard seeds & cumin seeds splutter, add curry leaves.
Add chopped Onions and saute them until they turn translucent.
Add green chillies.
Add Turmeric Powder. Saute until the raw smell goes off.
Add in chopped potatoes and saute for a few mins.
Add 3/4th cup of water and season it with salt.
Close the pressure cooker and cooker for 3 whistles.
Once the pressure is released, open the cooker and mash the potato masala with a ladle or a masher.