Fill a large bowl with cold water. Cut the eggplant into 2cm cubes, and immediately place into the water to keep from turning brown. Pour the oil and mustard seeds into a pot, and place over medium heat. Cook until the seeds begin to pop, being careful not to burn.
Stir in the onion, and cook until tender. Sprinkle in the chilli powder, turmeric, cumin, coriander, garlic, ginger, garam masala and caraway seed; cook for a minute or two to release the flavour.
Drain the eggplant, add it to the pot and fry it for a few minutes. Pour in the potatoes and soy sauce, then add enough water to cover. Turn the heat to high, bring to a boil, then turn heat to low and simmer for approximately 30 minutes until potatoes have cooked. Serve.