Heat the oil for the tempering in a large, heavy-based frying pan over medium heat, add all the ingredients for the tempering, and stir-fry for 1 minute, or until the mustard seeds start to splutter.
Add the sweet potatoes, chillies, turmeric, and season with salt.
Pour 125ml / 4 1/2fl oz (1/2 cup) water into the pan, then reduce the heat, cover, and simmer for about 10-15 minutes, or until the sweet potatoes are cooked and all the water is completely absorbed.
Sprinkle in the poriyal powder, mix well and remove from the heat.