In a Kadai or wok, heat oil and butter. Add the onions, and saute till they become transculent. Add the tomatoes and fry till they wilt. Sprinkle in the salt and red chillie powder. Stir in peas and let them cook till tender. If using frosted peas, defrost before use. Blend in the mashed eggplant into the masala and stir well. Cook on medium heat for around 10 minutes.