Heat the oil in a wok or kadhai.
Add the panch phoron and, when the seeds sputter, add the green chillies. Stir-fry for about 30 seconds.
Add the potatoes and stir-fry them for about two minutes.
Now add the poppy-seed paste, salt to taste, and about 1/2 cup water. Mix everything together well and when the mixture starts to simmer, place a tight-fitting lid on the wok, turn the heat down to medium-low, and let the mixture cook until the potatoes are tender. Stir occasionally. If the water has evaporated before the potatoes are cooked, add a few teaspoons more at a time so the potatoes and poppy don't burn.
Once the potatoes are cooked, turn off the heat. Garnish with coriander, if you like, and serve hot.