Dry-roast the coriander seeds, dried red chilies, cumin seeds, fennel seeds, and black peppercorns in a skillet over medium heat until fragrant and slightly darkened, 2–3 minutes; cool, then grind coarsely to make the kadai masala.
Prep the produce: clean and quarter the mushrooms, finely chop the onion and tomatoes, and cut the green bell pepper into 1-inch squares.
Stir in the tomatoes and cook, stirring often, until thick, glossy, and the oil starts to separate, 8–10 minutes.
Add the turmeric powder, kashmiri red chili powder, and about two-thirds of the kosher salt along with half of the ground kadai masala; cook 1 minute to bloom the spices.
Add the mushrooms and green bell pepper; toss on high heat until coated and just beginning to soften, 2–3 minutes. Cover and cook until the mushrooms are tender and release their juices, 5–7 minutes.
Uncover, pour in the water to form a thick, spoonable gravy, and simmer until the vegetables are well coated and glossy, 3–5 minutes. Stir in the remaining kadai masala and the kasuri methi; adjust seasoning if needed and cook 1 minute more.
Turn off the heat and garnish with cilantro and ginger. Rest 2 minutes to settle the flavors, then serve hot with roti, naan, or steamed rice.