Heat a large nonstick pan over medium-high heat. Add the oil and once shimmering, add the diced onion, ginger paste, and minced garlic. Stir occasionally and cook until golden, about 3 to 6 minutes.

Add all the spices to the pan and stir constantly for 1 to 3 minutes until the spices bloom and smell deeply fragrant.

Stir in the canned tomatoes and scrape the bottom of the pan. Let the mixture cool slightly, then transfer to the slow cooker.

Trim large pieces of fat and gristle from the chicken thighs, leaving some fat for flavor. Add chicken to the slow cooker, stir to coat in the sauce, then arrange smooth side down in a single layer.

Cover and cook on HIGH for 2.5 to 4 hours or LOW for 4 to 6 hours until chicken reaches an internal temperature of 165 degrees F.

Remove the chicken and chop into bite-sized pieces. Let the sauce cool 5 minutes, then puree it smooth using an immersion blender. If using a standard blender, hold the lid down with a folded towel and blend in short bursts.

Return the blended sauce to the slow cooker on the warm setting. Stir in the sliced butter and heavy cream until fully combined and silky.

Add the chopped chicken back into the sauce. Stir in cilantro if using. Taste and adjust seasoning as needed.

Serve hot over cooked rice with warm naan on the side.