For the masala:
First make the masala. Dry-roast the peppercorns, coriander, clove, cardamom, cinnamon, mustard seeds and cumin in a heavy-based frying pan over medium heat for about 1 minute, being careful they do not burn. Allow to cool, then grind together into a powder using a coffee-grinder or a pestle and mortar. Heat the oil in a saucepan, add the onion and fry until translucent. Add the green chillies, ginger and garlic and allow them to soften. Add the masala, turmeric and vinegar. Stir well and add the coriander leaves. Add the water and simmer for 5 minutes. Puree the mixture in a blender or food processor and return to the stove. Add the mushrooms and simmer until they are tender, about 5 minutes. Add salt to taste and serve hot garnished with slivers of fresh red chillies.