Heat 1 tablespoon of oil or ghee in a wide pan over high heat. Add the cleaned and cut mushrooms. Sauté for 4-5 minutes without crowding until they release their water, absorb it back, and turn golden brown on the edges. Remove them from the pan and set aside.
In the same pan, add another tablespoon of oil over medium heat. Add the cumin seeds and let them crackle. Add the chopped onions and cook slowly, stirring often, for 8-10 minutes until they turn sweet and deep golden-brown.
Stir in the ginger-garlic paste and slit green chilies. Sauté for 1 minute until fragrant. Reduce heat to low, add the turmeric, Kashmiri red chili powder, coriander powder, and salt. Stir constantly for 30 seconds to toasted spice aromas.
Pour in the fresh tomato puree. Cook over medium heat for 6-8 minutes, stirring occasionally, until the mixture reduces into a thick paste and you see drops of oil separating around the edges.
Pour in 1/2 cup of warm water and stir to form a smooth gravy. Bring it to a simmer, then gently stir in the sautéed mushrooms. Cover and simmer on low heat for 5 minutes to let the mushrooms absorb the spices.
Uncover, reduce heat to low, and stir in the heavy cream and garam masala. Rub the kasuri methi between your palms, sprinkle it over the curry, stir once, and turn off the stove. Serve warm.