Wash the green peas and soak the dry green peas in enough water for about 4 to 5 hours or overnight.
After soaking, you can drain the water and lightly rinse the green peas and keep it aside. If using fresh or frozen peas, you can skip this step of soaking, just wash and use directly.
Peel the potatoes and dice evenly in medium sized cubes, keep aside with green peas.
Heat oil in a pressure cooker and add the mustard seeds.
When the mustard seeds start spluttering, add the green peas and diced potatoes, lightly saute for 1 to 2 minutes.
Add the salt and spices, mix everything well and saute for another 1 minute.
Add the water and mix well.
Close the pressure cooker using the lid and once the steam begins to release, cook in medium heat for about 7 to 8 minutes or until 2 to 3 whistles have come. Make sure you us the safety valve of the pressure cooker and follow all directions in the manual as they are important for your safety while using the pressure cooker.
After 7 to 8 minutes of cooking, you can switch off the stove and allow the pressure cooker to cool down.
Once the pressure cooker is cool down, you can open the cooker and see if the vegetables (peas and potatoes) are cooked well until soft. You can mix everything well and heat the mixture for another 1 minute so everything will blend nicely to form the green peas masala gravy. If you need more gravy, you can add more water at this point. You may also use a little coconut milk instead of water, if you want more gravy. Serve the potato green peas masala hot with chapathi or dosa, idli or rice!