In a large plate, combine chopped onions, green chillies, coriander leaves, red chilli powder, black salt, and salt. Mix everything well.
Place a dough ball over the onion mixture and gently spread it out into a circular shape using your fingers. To prevent the dough from sticking to your fingers, apply water frequently while spreading.
Once the dough is flattened, apply a small amount of water on top. Carefully lift the roti and place it, water-side down, on a preheated tawa. The water will help the roti stick to the tawa. Cook for 1 minute.
Flip the tawa and cook the roti until it's charred and cooked through.
Alternatively, you can remove the roti from the tawa and cook it directly over the flame for a few seconds to add a smoky flavour.
Transfer the masala pyaz ki roti to a serving plate, apply a generous amount of white butter, and serve hot with chole and aam ka achaar.