Heat oil in a wok or large frying pan (with deep sides). Saute ginger and garlic till light brown, and then add the onions.
Once the onions are light golden, add green chilies followed by spices (red chili powder, turmeric and salt).
Saute for a minute and then add the crushed tomatoes, along with about ½ cup of water. Cook for about 5 minutes till the oil starts releasing from the edges.
Pour the tomato mixture into a blender and puree till smooth.
Pour the tomato puree back into the pan, and bring it to a low simmer. Add the cream and butter.
Once the butter is melted, add chopped capsicums and sliced mushrooms. Allow to cook for about 2 minutes, and then turn off the heat.
Garnish with coriander leaves and cream or butter. Serve with steamed rice or naan.