01 - Whisk together yogurt, lemon juice, ginger paste, garlic paste, cumin, coriander, garam masala, chili powder, and salt in a large bowl. Add chicken pieces and toss to coat completely. Cover and refrigerate for at least 1 hour, preferably overnight for maximum flavor penetration.
02 - Preheat oven broiler or grill pan to high heat. Arrange marinated chicken on a foil-lined baking sheet or skewers. Broil or grill for 8 to 10 minutes, turning once halfway through, until pieces develop light charring and are cooked through. Remove and set aside.
03 - Melt butter with oil in a large skillet over medium heat. Add chopped onion and sauté for 5 to 6 minutes until golden brown. Stir in minced garlic and ginger paste; cook for 1 minute until fragrant.
04 - Pour in crushed tomatoes, then add cumin, coriander, paprika, garam masala, sugar, and chili powder. Stir well and simmer for 10 to 12 minutes, stirring occasionally, until sauce thickens and oil begins to separate on the surface.
05 - Transfer the sauce to a blender and purée until completely smooth, or use an immersion blender directly in the pan. Return the smooth sauce to the skillet and bring back to a gentle simmer.
06 - Add grilled chicken pieces to the sauce. Pour in heavy cream and sprinkle with dried fenugreek leaves. Simmer gently for 8 to 10 minutes until flavors meld together and chicken is tender. Season with salt to taste.
07 - Transfer to a serving dish and garnish generously with chopped fresh cilantro. Serve immediately while hot, accompanied by basmati rice or warm naan bread.