First of all you need a pressure cooker and a tiffin box which fits in the pressure cooker. If not a tiffin box, then a bowl with a proper lid will do.
Make sure than you really make fine slices of the onions and the potatoes, especially the potatoes since they take longer to cook than the prawns and you wouldn't want raw potatoes.
Now, take the bowl which you will place inside the pressure cooker and add all the ingredients.
Add a tbsp of water and and mix everything nicely. Make sure there is enough seasoning. Slit the green chilies if you want an additional kick.
Fill the pressure cooker with 2 -3 cups of water. Now cover your bowl or tiffin box and place inside the pressure cooker. Note that the level of the water should be little less than half of the tiffin box or bowl.
Close the lid of the pressure cooker and let it cook for 10 minutes or 2 -3 whistles.
In case you don't want to use a pressure cooker, then you can also simply place the covered bowl inside a wok filled with water. Cover the entire wok and let it cook on low flame for about 20 minutes.
Serve it hot with some steamed rice.