Slice the eggplant into 1/4 inch thickness. Cut the slices into 2 inch pieces. Deep fry till golden brown and set a side. Meanwhile slice the onion, garlic, tomatoes, and green chillie and mix in a bowl with vinegar, chillie powder, cumin powder and salt. Heat 1 tbsp of oil in a skillet. Add the onion mixture and curry leaves. Fry for about 5 minutes. Next add the cashew nuts, fried eggplant, sugar and ground mustard and fry for another minute or so. Serve warm. This goes well with rice. You can substitute the ground mustard with mustard paste.