In a large bowl, combine Greek yogurt, lemon juice, ginger-garlic paste, turmeric, garam masala, cumin, coriander, smoked paprika, chili powder, salt, and oil. Mix well.

Cut the chicken into bite-sized chunks and coat them thoroughly in the marinade. Cover and refrigerate for at least 1 hour (preferably overnight for maximum flavor).

Heat a grill pan, skillet, or broiler over medium-high heat. Drizzle a little oil on the pan and sear the marinated chicken for about 4-5 minutes on each side until slightly charred and cooked through. Set aside while preparing the sauce.

In a deep skillet or saucepan, heat butter and oil over medium heat. Add chopped onions and sauté until golden brown (about 5-7 minutes). Stir in garlic and ginger, cooking for another minute until fragrant.

Add cumin, coriander, garam masala, turmeric, smoked paprika, and chili powder. Stir for 30 seconds to toast the spices. Mix in tomato paste and cook for 2 minutes, then pour in the crushed tomatoes. Simmer for 10 minutes, stirring occasionally, until the sauce thickens.

Reduce heat and slowly stir in heavy cream. Add sugar (if using) and season with salt and pepper to taste.

Add the grilled chicken pieces to the sauce, stirring to coat evenly. Simmer for 5 more minutes to let the flavors meld. Stir in lemon juice and garnish with fresh cilantro.

Serve hot with basmati rice, naan, or roti.