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 (see Note 5).
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.
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Notes
Note 1: Pre-prepared ginger and garlic paste from the refrigerated section saves time.Note 2: Garam masala is critical for flavor—don’t skip it!Note 3: Chicken thighs are ideal for this dish. They stay juicy and tender, unlike chicken breasts, which can become tough.Note 4: Serving Suggestions: Use any type of rice, even microwaveable, for ease. Warm naan by spraying with cooking oil and lightly grilling over medium heat until charred, flipping once. Keep warm wrapped in a towel. You can also warm naan in a skillet, toaster, or microwave under a damp towel.Note 5: Blending Tip: Let the sauce cool slightly before blending to avoid overflow or burns. If using a blender not designed for hot liquids, secure the lid with a towel to contain steam.Storage: Store in an airtight container for up to 3–5 days. Reheat gently on the stove, adding water or cream to adjust the sauce thickness.