Combine the Base: In your slow cooker, mix together the tomato sauce, chicken broth, tomato paste, ginger, garam masala, curry powder, ground cumin, salt, black pepper, and minced garlic until well combined and smooth.
Add the Chicken: Stir in the diced chicken pieces and onion until everything is evenly coated in the spiced tomato mixture.
Cook on Low: 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 tender and cooked through.
Stir in the Cream: About 15 minutes before serving, remove the lid and stir in the butter cubes and heavy cream, mixing until the butter is fully melted and the sauce becomes silky and rich.
Taste and Adjust: Add lime juice if desired and taste the sauce, adjusting salt, spices, or lime juice to your preference.
Prepare Your Base: Spoon the butter chicken over steamed basmati rice, jasmine rice, or naan bread as your preferred serving vessel.
Ladle the Sauce: Make sure each portion gets plenty of the creamy, spiced sauce over the chicken and rice.
Garnish: Top with fresh cilantro if desired and serve immediately while hot.