01 - In a large mixing bowl, combine the yogurt, lemon juice, cumin, garam masala, turmeric, salt, and coriander. Add the chicken pieces and toss thoroughly to coat evenly. Allow to marinate for at least 15 minutes at room temperature, or cover and refrigerate overnight for deeper flavor penetration.
02 - Melt 1 tablespoon of butter in a skillet over medium heat. Cook the chopped onion, stirring occasionally, until golden and softened, about 4 minutes. Add the minced garlic and ginger, stirring constantly for 1 minute until fragrant. Transfer the entire mixture to the crockpot.
03 - Add the marinated chicken along with all of the marinade to the crockpot. Pour in the crushed tomatoes, paprika, cayenne pepper, sugar, and the remaining 1 tablespoon of butter. Stir everything together until well combined.
04 - Cover the crockpot and cook on the low setting for 6 hours, or on high for 3 to 4 hours, until the chicken is fall-apart tender and the sauce has thickened.
05 - Stir the heavy cream into the crockpot until fully incorporated. Taste and adjust the salt and spice level as needed. Cook uncovered for an additional 10 minutes to allow the flavors to meld and the sauce to reach the desired consistency.
06 - Ladle into bowls and garnish generously with chopped fresh cilantro. Serve alongside steamed basmati rice or warm naan bread.