Marinate the Chicken: In a large bowl, combine all marinade ingredients. Mix well to ensure the chicken is evenly coated. Cover and refrigerate for at least 2 hours, or preferably overnight.

Prepare the Sauce: Heat butter and oil in a large, heavy-bottomed pot or Dutch oven over medium heat. Add chopped onion and cook until softened and lightly browned, about 5-7 minutes. Add ginger-garlic paste and sauté for another minute until fragrant.

Bloom the Spices: Add garam masala, cumin powder, and red chili powder to the pot. Cook for 30 seconds, stirring constantly, until fragrant. Be careful not to burn the spices.

Add Tomatoes and Simmer: Pour in the crushed tomatoes and bring to a simmer. Reduce heat to low, cover, and cook for 20-25 minutes, stirring occasionally, until the tomatoes have broken down and the sauce has thickened slightly.

Cook the Chicken: While the sauce simmers, preheat a grill or large skillet over medium-high heat. Remove the chicken from the marinade and grill or sear in batches, ensuring not to overcrowd the pan. Cook until the chicken is cooked through and lightly charred, about 5-7 minutes per batch.

Combine and Finish: Add the cooked chicken to the simmering tomato sauce. Stir in the heavy cream, cashew paste (if using), and sugar (or honey). Season with salt to taste. Simmer for another 10-15 minutes, stirring occasionally, to allow the flavors to meld together.

Final Touches: Stir in the crushed dried fenugreek leaves (kasuri methi). Garnish with fresh cilantro leaves before serving. Serve hot with naan bread or rice.