Place the cut chicken pieces into the basin of your slow cooker.
Add the crushed tomatoes, chicken broth, chopped onion, minced garlic, grated ginger, tomato paste, garam masala, turmeric, cumin, paprika, cayenne pepper (if using), salt, and black pepper to the slow cooker.
Stir all ingredients together until the chicken is well coated in the sauce mixture.
Cover the slow cooker and cook on low for 6 to 8 hours, or on high for 3 to 4 hours, until the chicken is very tender.
About 30 minutes before serving, remove the lid. In a small bowl, mix the Greek yogurt or sour cream with the heavy cream. Stir this mixture into the slow cooker.
Add the butter and dried fenugreek leaves. Stir gently until the butter melts and the sauce is smooth. Do not let the mixture boil after adding the dairy.
Taste and adjust salt if needed. Serve hot, garnished with fresh cilantro.
Notes
For best texture, shred the chicken slightly with two forks before stirring in the cream mixture in the final step.
If you prefer a smoother sauce, use an immersion blender carefully in the slow cooker before adding the cream.
Serve this curry over basmati rice or with warm naan bread.