1. Heat oil in a non-stick pan.
2. Add cumin seeds and when they change colour add onions and sauté till the onions are soft. Add tomatoes, salt and red chilli powder and mix. Add sugar and mix. Slit the buns without cutting through.
3. Heat a non-stick tawa. Apply a little butter and some of the onion-tomato mixture on the inner side of the buns. Place them on the tawa, drizzle a little oil or butter. Press them lightly, turn over and roast till both sides are crisp.
4. Serve hot.