Cook the potatoes in a large pan of boiling water for about 20 minutes, or until soft. Drain and allow to cool, then peel off the skins and transfer the potatoes to a large bowl.
Add the chillies, ginger, chilli powder, mustard oil, and season with salt. Use your hands to mix everything together and mash well.
Chargrill the tomatoes over the open flame of a gas cooker (gas stove top) for a few minutes, or until the skins are blackened, then peel off the skins, transfer the flesh to a bowl, and mash with a spoon.
Alternatively, put the tomatoes in a large heatproof bowl of boiling water for 30 seconds, then plunge them in cold water, drain, allow to cool slightly, and remove the skins.
Heap the mashed potato mixture on a serving plate, arrange the mashed tomato decoratively around and garnish with coriander.
If using, heat the oil for the tempering in a small frying pan (skillet) over medium heat, add the cumin seeds, and stir-fry for about 1 minute, or until they start to splutter. Pour over the mashed potato and tomato.