Heat the butter and the oil in a wok or pan and add the cumin seeds. Once the seeds begin to crackle, add the onion and sauté for 4-5 minutes, stirring occasionally.
Add the garlic paste and cook for another 4 minutes.
Add the green pepper and sauté for another few minutes.
Now, add the chopped tomatoes and cook for a further 3-4 minutes.
Add the Rajah Pav Bhaji Masala and the salt. Cook for another 2 minutes. Add all the vegetables, mix well and cook with a cup of water. Mash the vegetables until they become a pulp-like consistency.
Mix well and cook for another 2-3 minutes. Top the bhaji with butter just before serving.
To serve with the buns, heat 8 tsp butter in a pan. Add the sliced buns and cook until browned on both sides. Place the hot bhaji on the sliced pav, top with coriander and chopped onions. Serve with chopped onions and a lemon wedge.