Cook the yam in a large pan of boiling water for about 10 minutes, or until tender. Drain and cut into 4-cm / 1 1/2-inch cubes.
Heat the ghee in a large, heavy-based pan over medium heat, add the yam and fry for about 5 minutes, or until golden. Remove with a slotted spoon and set aside.
Add the asafoetida and cumin seeds to the pan and stir-fry for 1-2 minutes. Stir in the turmeric, chilli powder, coriander, salt, and tomatoes and cook for a further 2-3 minutes.
Add the fried yam and peas, then pour in 250ml / 8fl oz (1 cup) water, reduce the heat to low, cover, and cook, stirring occasionally, for 15-20 minutes, or until the vegetables are tender.
Add the yoghurt, garam masala, and amchoor and, cook for a further 10 minutes.