rub eggplant pieces with little salt and either fry them in a pan till cooked or you can microwave those brushing with little oil till cooked and goldenish.
Arrange the pieces in a dish
To prepare the sauce, paste together the tomato puree and mustard paste / seeds / or ground mustard. In the same pan add remaining oil and add the paste.
Add salt, green chilly and cook till oil separates adding little water in between. (You can add water per your required consistency.) Pour the Mustard and tomato sauce over the fried eggplant and serve with rice, roti, bread.