In a large bowl, mix together Greek yogurt, lemon juice, turmeric, garam masala, cumin, coriander, smoked paprika, chili powder, and oil. Add the chicken breasts and coat well. Cover and refrigerate for at least 1 hour, preferably overnight.

Preheat grill or stovetop pan over medium-high heat. Add marinated chicken in a single layer and sear each side for 4-5 minutes until slightly charred and cooked through. Remove and set aside.

In a large skillet, heat butter or ghee over medium heat. Add diced onions and sauté until golden brown for 5-7 minutes. Incorporate garlic and ginger, cooking for an additional minute.

Sprinkle in remaining spices and toast for 30 seconds. Add tomato paste and crushed tomatoes; mix well. Simmer on low for about 10 minutes until it thickens.

Lower the heat and pour in heavy cream or coconut milk, stirring until integrated. Cook for 3-5 minutes to create a velvety texture. Adjust seasoning, adding sugar if needed.

Add cooked chicken into the sauce, stirring to coat it. Simmer for another 2-3 minutes. Finish with lemon juice and cilantro before serving with basmati rice or naan.