For preparing the sandwich, heat oil in a frying pan, add chopped onions to it.
When the onion turns brown, add chopped tomatoes along with green chilli, chilli powder and salt. Add chopped mushrooms as well and mix the contents. Let it saute for a few minutes.
Meanwhile fry paneer pieces.
Add the fried pieces into the previously prepared mixture and pour the sauce.
Cut open the buns. Butter the sides and stuff the buns with the paneer-mushroom masala.