1. Cut the chicken thighs into bite-sized pieces and season with salt and pepper.
2. In a large bowl, mix together the Greek yogurt, garam masala, turmeric, paprika, cumin, coriander, and cayenne pepper. Add the chicken pieces and stir until well coated. Cover and refrigerate for at least 30 minutes or up to overnight.
3. In a large skillet over medium heat, melt the ghee or butter. Add the diced onion and cook until softened, about 5 minutes. Add the minced garlic and grated ginger and cook for another minute.
4. Add the marinated chicken to the skillet and cook until browned on all sides, about 8-10 minutes.
5. Stir in the tomato sauce and heavy cream. Reduce heat to low and simmer for 10-15 minutes until the chicken is cooked through and the sauce has thickened.
6. Serve hot, garnished with fresh cilantro.
Enjoy your delicious keto butter chicken!