Put the potatoes into a heavy-based pan, add half the turmeric, and season with salt. Pour in enough water to cover, then bring to a boil and cook for about 12-15 minutes, or until soft. Drain and set aside.
To make the masala, heat a Tawa, griddle, or small, heavy-based frying pan (skillet) over very low heat, add the coriander seeds, and stir-fry for about 3 minutes, or until fragrant and roasted. Remove from the pan and roast the cumin seeds, black peppercorns, dried red chillies, green cardamom pods, cloves and cinnamon separately in the same way.
Transfer all the roasted spices, turmeric and lemon juice to a blender and process until ground. Pass through a sieve (strainer) and set aside.
Heat the ghee in a large, heavy-based pan overmedium heat, add the mustard and cumin seeds, and stir-fry for about 1 minute, or until they start to splutter.
Add the potatoes and season with salt, then stir-fry for 2 minutes. Add the masala and stir-fry for a further 2 minutes, before removing from the heat and adjusting the seasoning, if necessary.
Heat the oil for the tempering in a small frying pan over medium heat, add the sliced onion and fry for about 5-7 minutes, or until golden, then add the curry leaves. Stir and spread over the potatoes.