Heat oil in a pressure cooker. Splutter the mustard seeds and fry the curry leaves.
Add the onion and green chilies and saute till wilted.
Add the ginger and cook till raw smell is gone.
Add the spice powders and toss.
Add the chopped potatoes, coconut milk and salt. Close the pressure cooker and cook for 2-3 whistles or until done. Allow pressure to go by itself.
Open, adjust seasoning and enjoy hot with pooris.