Place frying pan on high heat, add cumin seeds, when fragrant (10-20 seconds) take off heat and set aside.
Remove skin from potatoes and dice into 1/4 inch cubes;
Cut off stem of aubergine and dice into 1/4 inch cubes, with the skin on;
Place pan (approximately 10-in diameter) on medium-high heat and pour in a glug of oil;
Add ginger, garlic and mustard seeds-careful, it will splatter, have your pan lid handy (or some aluminium foil). Let it splatter for 30 seconds so the garlic and ginger is browned.
Carefully remove lid (I take the pot off the burner and let it rest for at least 10 seconds before I remove the lid) and add potatoes.
Turn the heat to medium;
Saute for five minutes and add a tablespoon or more of water to prevent the potatoes from sticking to the bottom of the pot.
Add aubergine and continue to stir. Add cayenne, turmeric and salt to taste. Add a few more tablespoons of water (again, to prevent aubergine or potatoes from sticking to the bottom of the pan) and then add tomatoes.
Turn the heat to low-medium, place lid on top and allow to steam for 15 minutes. Check at the 10 minute mark.
When ready, sprinkle with roasted cumin and fresh chopped cilantro.
Eat with rice, naan, chapati or toasted bread :)