In a large bowl, combine Greek yogurt, lemon juice, ginger-garlic paste, turmeric, garam masala, cumin, coriander, smoked paprika, chili powder, salt, and oil. Cut chicken into bite-sized pieces and coat in the marinade. Cover and refrigerate for at least 1 hour (preferably overnight).

Heat a skillet over medium-high heat, add oil, and sear marinated chicken for 4-5 minutes on each side until charred. Alternatively, bake at 400°F for 15-20 minutes.

Melt butter or ghee in the same skillet over medium heat. Sauté onions until golden brown for 5-7 minutes. Add garlic and ginger, stirring for another minute.

Sprinkle in the remaining spices, stirring for 1-2 minutes. Add tomato paste and crushed tomatoes, simmer for 10 minutes until thickened.

Reduce heat, stir in heavy cream or coconut milk, and let simmer for 5 minutes. Adjust seasoning as required.

Add seared chicken back into the sauce and simmer for another 5 minutes. Finish with a squeeze of lemon juice and garnish with cilantro.