Soak and grind the white paste: Add the cashews and poppy seeds to a small bowl, cover with warm water, and soak for 30 minutes. Drain and grind with the green chilies and 2-3 tablespoons of fresh water into a very fine, silky paste. If your blender leaves any graininess, pass the paste through a fine-mesh sieve.
Marinate the chicken: In a large bowl, whisk together the yogurt, ginger garlic paste, white pepper powder, lemon juice, and salt. Add the chicken pieces and turn until every piece is coated. Cover and refrigerate for at least 1 hour, or up to overnight.
Temper the whole spices in ghee: Heat the ghee and oil in a heavy-bottomed pot or Dutch oven over medium heat. Add the green and black cardamom, cloves, cinnamon, bay leaf, and mace. Sizzle for 30-45 seconds until fragrant.
Soften the onions (no browning!): Add the thinly sliced onions and cook on medium-low heat for 8-10 minutes, stirring often, until completely soft and translucent but NOT browned. If the onions start catching color, lower the heat and add a splash of water.
Cook the chicken in its marinade: Raise the heat to medium-high and add the marinated chicken along with all the marinade. Cook for 8-10 minutes, turning the pieces, until the chicken is lightly seared and the yogurt has thickened and coats the meat. Keep stirring so the yogurt doesn't split.
Add the white paste and simmer: Stir in the ground cashew-poppy paste and cook for 3-4 minutes until the raw smell disappears and the gravy turns glossy. Pour in the hot water, season with salt, and bring to a gentle simmer. Cover and cook on low heat for 25-30 minutes, stirring occasionally, until the chicken is fall-off-the-bone tender and the gravy has thickened. Take the pot off the heat, then stir in the kewra water, saffron milk, cream (if using), the final tablespoon of ghee, and the grated nutmeg. Rest 5 minutes, garnish with fried onions, and serve.