Marinate: In a large bowl, whisk the yogurt, lemon juice, garam masala, cumin, paprika, salt, garlic, and ginger. Add the chicken and turn to coat. Cover and refrigerate for at least 30 minutes or up to 12 hours.
Char the chicken: Position an oven rack 6 inches from the broiler and heat the broiler to high. Arrange the chicken on a foil-lined rimmed baking sheet. Broil for 5 to 7 minutes per side, until cooked through with charred edges. Set aside.
Soften the onion: Melt the butter with the oil in a large deep skillet over medium heat. Add the onion and cook for 6 to 8 minutes, stirring often, until soft and golden.
Bloom the spices: Stir in the garlic, ginger, and tomato paste and cook for 2 minutes. Add the garam masala, cumin, turmeric, and cayenne and cook for 30 seconds, until fragrant.
Simmer the sauce: Pour in the crushed tomatoes and 1/2 cup water. Simmer for 12 to 15 minutes, until thickened.
Blend until silky: Blend the sauce with an immersion blender until completely smooth (or carefully in batches in a countertop blender), then return it to the pan.
Finish: Reduce the heat to low and stir in the cream and crushed kasuri methi. Add the chicken and any accumulated juices and simmer for 5 minutes.
Serve: Taste and adjust the salt. Shower with cilantro and serve hot over basmati rice with warm naan.