01 - In a large mixing bowl, whisk together the yogurt, lemon juice, ground cumin, garam masala, turmeric, kosher salt, and ground coriander until smooth and well combined.
02 - Add the chicken thigh pieces to the marinade, tossing thoroughly to coat every piece evenly. Cover and let rest at room temperature for at least 15 minutes, or refrigerate overnight for more pronounced flavor.
03 - Melt 1 tablespoon of the butter in a large skillet over medium heat. Add the chopped onion and cook, stirring occasionally, until golden and softened, about 4 minutes. Stir in the minced garlic and ginger, cooking for 1 additional minute until fragrant. Transfer the entire mixture to the slow cooker.
04 - Add the marinated chicken along with all of the marinade to the slow cooker. Pour in the crushed tomatoes, then add the paprika, cayenne pepper, sugar, and the remaining 1 tablespoon of butter. Stir everything together until evenly combined.
05 - Secure the lid and cook on the LOW setting for 6 hours (or on HIGH for 3 to 4 hours), until the chicken is fork-tender and cooked through.
06 - Stir the heavy cream into the sauce, then taste and adjust salt and spice levels as needed. Leave the lid off and continue cooking for 10 minutes to allow the sauce to thicken slightly. Ladle into bowls, garnish with fresh chopped cilantro, and serve alongside steamed basmati rice or warm naan.