Cook the potatoes in a pan of boiling water for about 20 minutes, or until soft. Drain and allow to cool, then peel off the skins. Cut the potatoes into 1-cm / 1/2-inch cubes.
Put all the ingredients for the spice paste in a blender with a little water, if necessary, and process to make a smooth paste. Set aside.
Heat the oil for the tempering in a kadhai or deep, heavy-based frying pan (skillet) 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 onions and stir-fry for 5 minutes, or until golden.
Mix in the chillies, ginger, tomatoes, and turmeric, and season with salt. Fry for about 1-2 minutes.
Pour 250ml / 8fl oz (1 cup) water into the pan, stir, then reduce the heat, cover and simmer for about 5 minutes.
Mix in the spice paste and potatoes and simmer, uncovered, for a further 5 minutes, or until well blended. Pour in the coconut milk and simmer for about 1-2 minutes.