Grind mustard seeds and green chili into a smooth paste using 2 tbsp water.
In a heavy pot, combine raw prawns, thinly sliced potato batons, mustard paste, turmeric, salt, mustard oil, and water.
Mix thoroughly, cover pot tightly with a lid, and cook on low flame for 12 minutes.
Uncover and cook for 2 minutes to let gravy reduce slightly before serving with rice.