Heat oil in a skillet or wok. Add the mustard seeds to it, then the cumin. As soon as the mustard seeds crackle, add half the cilantro, curry leaves and slit green chili peppers.
Stir in the onions with a pinch of salt and saute them until they are a rich golden brown. Onions fried this way will melt into the gravy and help thicken it, so take your time and don't leave them half-cooked.
Add the ginger garlic paste, stir it in and saute for a minute.
Add the tomatoes along with the turmeric, ground fennel and ground coriander. Mix well and let the tomatoes cook, stirring frequently, until they are pulpy and very soft and have acquired a deeper color
Add the coconut milk and cook the curry for 4-5 minutes until it's a rich reddish-brown in color.
Add the chopped mushrooms and stir them in. Let the curry come back to a boil, then stir in the garam masala, ground black pepper and salt to taste. Turn off the heat.
Stir in the remaining cilantro, then serve hot.