In a bowl mix together potatoes, onion, carrots and yogurt. Mix well. Heat oil in a non stick pan, add cumin seeds and green chillies, allow it to splutter. Add to the yogurt and add salt and mix well.
In a pressure cooker heat butter, add cardamom, cloves, cinnamon, star anise and saute well till fragrant. Add rice and saute well for a minute. Add onion, ginger-garlic paste, green chillies and tomatoes. Mix well. Add mushrooms and saute again.
Add salt, coconut milk, water and chopped mint and coriander leaves. Mix well. Allow it to cook till three whistles.