Bring 750ml / 1 1/4 pints (3 1/4 cups) water to a boil in a large, heavy-based pan, add the colocasia, cover, reduce the heat to medium and cook for 20 minutes.
Drain, allow to cool, then peel off the skins and cut into 1-cm 1/2-inch thick slices.
Put the yoghurt, gram flour, ginger, and garlic pastes, chilli powder, turmeric, and 250ml / 8fl oz (1 cup) water in a large bowl. Season with salt and mix together. Set aside.
Heat the oil in another large, heavy-based pan over medium heat, add the cloves, cardamom pods, and ajwain seeds, and stir-fry for about 1 minute, or until they start to splutter. Season with salt and add the garam masala, and stir-fry briskly for 15 seconds.
Add the colocasia and fry for about 10 minutes, or until there is no more stickiness and the colocasia turns light brown.
Reduce the heat and add the yoghurt mixture. Stir and cook for about 5 minutes.