Heat a large nonstick pan over medium-high heat. Add the oil, and once hot, sauté onion, garlic, and ginger, stirring occasionally, until golden (3–6 minutes). Add all the spices and stir constantly for 1–3 minutes until fragrant. Stir in the tomatoes, scraping the bottom of the pan, and let the mixture cool slightly. Transfer to the slow cooker.

Trim large pieces of fat and gristle from chicken thighs but leave some fat for added flavor. Add the chicken to the slow cooker and stir to coat in the sauce. Arrange thighs in a single layer, smooth side down. Cover and cook on high for 2-1/2–4 hours or low for 4–6 hours, until the chicken reaches 165°F.

Remove the chicken and chop it into bite-sized pieces. Use an immersion blender (or transfer sauce to a regular blender) to purée the sauce until smooth.

Return blended sauce to the slow cooker. Stir in the butter and heavy cream until fully combined and smooth.

Add chopped chicken back into the sauce. Stir in cilantro (if using) and adjust seasoning as needed. Serve hot with rice and naan.