Parboil the mushrooms in a pan of boiling water for about 3-5 minutes.
Then drain and put into a bowl of cold water.
Drain again and squeeze out any excess water from the mushrooms.
Set aside.
Put the hung yoghurt, achari paste, ginger and garlic paste, mustard powder, kalonji seeds, roasted gram flour, mustard oil, egg, turmeric, garam masala and red chilli paste into a large bowl.
Then season with salt and mix well.
Add the mushrooms to the marinade and set aside for about 2 hours.
Prepare a tandoor or charcoal grill for a moderate heat, or alternatively preheat the grill (broiler) to medium.
Thread the mushrooms one by one onto several metal skewers and drizzle over the remaining marinade.
Roast in a tandoor at a moderate temperature, over a charcoal grill or under the hot grill for 4 minutes.