01 - In a large bowl, whisk together the Greek yogurt, lemon juice, garam masala, ground cumin, ground coriander, chili powder, ground turmeric, and salt until smooth. Add the chicken thigh pieces and toss thoroughly to coat every piece. Cover and refrigerate for at least 30 minutes, or ideally overnight for deeper flavor penetration.
02 - Melt 2 tablespoons of unsalted butter in a skillet over medium heat. Add the finely chopped onion, minced garlic, and grated ginger. Sauté, stirring frequently, until the onion turns soft and develops a deep golden color, approximately 5 minutes.
03 - Transfer the sautéed onion mixture into the crockpot. Add the marinated chicken along with all of the yogurt marinade, the canned tomato sauce, garam masala, smoked paprika, ground cumin, ground cinnamon, and cayenne pepper if using. Stir everything together until well combined.
04 - Secure the lid on the crockpot and cook on the LOW setting for 5 to 6 hours, or on the HIGH setting for 2.5 to 3 hours, until the chicken is fork-tender and the sauce has thickened and deepened in color.
05 - Approximately 20 minutes before serving, stir the heavy cream and remaining 2 tablespoons of unsalted butter into the slow cooker. Taste the sauce and adjust the salt and black pepper as needed. Allow the cream to warm through and integrate into the sauce.
06 - Ladle the butter chicken into serving bowls and garnish generously with fresh cilantro. Serve hot alongside steamed basmati rice or warm naan bread.