Combine Greek yogurt, lemon juice, ginger-garlic paste, turmeric, garam masala, ground cumin, ground coriander, smoked paprika, chili powder, and salt in a large mixing bowl. Add diced chicken, coat thoroughly, cover, and refrigerate for at least 30 minutes.

Heat a large pot over medium-high heat and add oil. Add marinated chicken in batches, searing for about 5-7 minutes per side until browned. Remove and keep warm.

In the same pot, reduce heat and add butter or ghee. Sauté chopped onion until golden, then add minced garlic and ginger, stirring until fragrant.

Stir in remaining spices and crushed tomatoes, simmer for about 10 minutes until thickened.

Reduce heat, return chicken to pot, add heavy cream, and simmer for another 15 minutes.

Taste sauce, adjust seasoning, garnish with cilantro and lemon juice before serving.