Marinate the chicken. In a large bowl, whisk the yogurt, lemon juice, ginger, garlic, salt, Kashmiri chili, garam masala, cumin, turmeric and 1 tbsp oil until smooth and evenly orange. Add the whole thighs and turn to coat every surface. Cover and refrigerate at least 1 hour, ideally 4 hours, and no more than 24 hours.
Brown the onions. Melt the ghee in a Dutch oven or wide heavy pot over medium heat. Add the onion, serrano (if using) and 1 tsp salt. Cook, stirring occasionally, 8 to 10 minutes, until the onion is soft and golden with darker edges.
Bloom the aromatics and spices. Stir in the ginger and garlic and cook 2 minutes, until very fragrant and just starting to stick to the pot. Add the garam masala, Kashmiri chili, paprika and coriander and stir constantly for about 1 minute, until the spices smell toasty but not burnt.
Simmer the tomatoes. Add the tomatoes and their juice, crushing them with a spoon, and scrape up the browned bits. Bring to a boil, then lower the heat and simmer uncovered, stirring now and then, 25 to 30 minutes, until reduced by about half and thick enough that a spoon dragged through it leaves a trail.
Blend and finish the sauce. Take the pot off the heat and blend until completely smooth with an immersion blender (or let it cool 5 minutes and blend in a countertop blender filled no more than halfway, lid vented and covered with a towel). Stir in the cream, then crush the kasoori methi between your palms over the pot. Return to the lowest heat. If it's thicker than heavy gravy, loosen with a splash of water.
Char the chicken (cast iron method). While the sauce simmers, turn on your vent fan and heat a 12-inch cast iron skillet or grill pan over medium-high heat for 5 minutes. Add 1 tbsp oil. Lift the thighs from the bowl, letting excess marinade drip off so only a thin coat remains. Cook in two batches without crowding, 3 to 4 minutes per side, until deep brown with blackened spots. The centers will still be underdone; that's intended.
Or char under the broiler. Set an oven rack 4 to 6 inches below the broiler and heat it on high. Arrange the thighs on a wire rack set over a foil-lined rimmed baking sheet and broil 4 to 6 minutes per side, until spotty and blackened at the edges. Watch closely after the first 3 minutes, since broilers vary a lot.
Finish the chicken in the sauce. Rest the charred thighs 5 minutes, then cut into 1-inch pieces. Add them, with any juices, to the sauce. Cover and simmer gently 8 to 10 minutes, until the thickest piece reaches at least 165°F (74°C) on an instant-read thermometer; thighs are most tender around 175°F (80°C).
Season and serve. Taste and adjust with salt, and with sugar if the tomatoes taste sharp. Scatter with cilantro and serve with basmati rice and warm naan.