Heat 3 tablespoons ghee or oil in a heavy-based frying pan (skillet) over medium heat, and working in batches fry the potatoes for 10-15 minutes, or until golden brown.
Remove from the heat. Place on kitchen paper (paper towel) to drain.
Add the bananas to the pan and fry for 2 minutes making sure not to scorch the bananas, then set aside.
Heat the remaining ghee or oil in a second large, heavy-based pan over medium heat. Add the asafoetida, cumin seeds, fenugreek, and kalonji seeds, and stir-fry for 1 minute, or until the seeds start to splutter.
Add the fried bananas, potatoes, and the remaining spices, then season with salt. Shake the pan a couple of times and pour in 250ml / 8fl oz (1 cup) water.
Reduce the heat, cover, and simmer for 10-15 minutes, or until the potatoes are soft. Add the yoghurt, cover, and simmer over very low heat for a further 10 minutes.