To make the marinade, put the yoghurt in a large bowl, add the spices and whisk well.
Heat the ghee in a large, heavy-based pan, add the cumin seeds, and stir-fry for about 1 minute, or until they start to splutter. Add the onion and stir-fry for about 5 minutes, or until it turns light golden. Add the ginger and garlic pastes and stir-fry for about 2-3 minutes, or until the onion is golden brown.
Remove from the heat, stir in the yoghurt mixture, then return to the heat and stir-fry for about 2-3 minutes, or until the oil starts to separate out and rise to the surface. Add the pureed tomato and the potatoes and stir-fry for a further 2-3 minutes, or until the oil separates out.
Pour in 1 litre / 1 3/4 pints (4 1/4 cups) water and season with salt. Bring to the boil, then reduce the heat to low and simmer, stirring frequently, for 10 minutes, or until the potatoes are three-quarters cooked.
Add the urad dal vadi and bring to the boil. Reduce the heat to low and simmer, stirring frequently, for 10 minutes, or until the potatoes are cooked and the sauce is a thin consistency.
Sprinkle over the ground fenugreek, stir and remove from the heat. Adjust the seasoning, if necessary. Garnish with chopped coriander.