In a mixing bowl, combine Greek yogurt, lemon juice, ginger-garlic paste, and spices. Cut chicken into bite-sized pieces and coat them with the marinade. Cover and refrigerate for at least 1 hour or overnight.

Heat a skillet over medium-high heat with vegetable oil. Add marinated chicken in batches, sear for 5-7 minutes until golden brown, and set aside.

In the same skillet, lower heat, add butter/ghee, followed by finely chopped onions. Cook for 5-7 minutes until golden, then add minced garlic and ginger for 1-2 minutes.

Stir in tomato paste and crushed tomatoes. Allow sauce to simmer on low heat for 10-15 minutes, stirring occasionally.

Mix in heavy cream or coconut milk, and season with salt, pepper, and optional sugar. Simmer for another 5 minutes.

Return seared chicken to the skillet, mix with the sauce, and simmer for 5 minutes.

Squeeze fresh lemon juice over the dish, garnish with cilantro, and serve hot with naan or rice.