Marinate the chicken. In a large mixing bowl, whisk together the yoghurt, lemon juice, cumin, coriander, smoked paprika, turmeric, garam masala, and salt until smooth and pale yellow. Add the chicken chunks and toss well, making sure every piece is coated. The yoghurt mixture should look thick and glossy, clinging to the chicken like a paste. Cover the bowl and refrigerate for at least 1 hour — overnight is even better.

Sear the chicken. Heat 2 tablespoons of oil in a large frying pan over high heat until it shimmers and you see faint wisps of smoke. Working in batches so you don't overcrowd the pan, add the chicken pieces in a single layer. Let them cook undisturbed for 3–4 minutes. You'll hear a vigorous sizzle, and the underside should turn a deep, charred brown with blackened spots. Flip and cook the other side for another 2 minutes. The exterior should look blistered and dark in places, while the inside remains just cooked through. Transfer to a plate and set aside.

Build the sauce base. Reduce the heat to medium. Add the remaining 1 tablespoon of oil and the butter to the pan. Once the butter has melted and starts foaming, tip in the diced onions. Stir frequently for 8–10 minutes, until the onions turn a deep golden brown — they should smell sweet and nutty, and have softened completely. Add the garlic and ginger, and cook for another minute until fragrant, stirring constantly so they don't burn.

Cook the tomato base. Stir in the tomato purée and cook for 2 minutes. It will darken to a rusty brick red and the oil will start to separate from the paste. Pour in the chopped tomatoes, scraping the bottom of the pan with your wooden spoon to release any browned bits — this is where so much flavour lives. Bring the sauce to a gentle simmer, then reduce the heat to low. Let it bubble softly for 10 minutes, stirring occasionally. The sauce will thicken and darken, and the raw, acidic tomato smell will mellow into something rich and savoury.

Blend the sauce (optional but recommended). For a silky-smooth masala, use a stick blender directly in the pan, or transfer the sauce to a countertop blender and blitz until completely smooth. Be careful with hot liquids — let the sauce cool for 5 minutes first. If you prefer a chunkier texture, skip this step and leave the onions as they are. Return the sauce to the pan over low heat.

Finish the masala. Stir in the double cream until the sauce turns a warm, peachy-orange colour. Add the seared chicken pieces along with any juices that have collected on the plate. Simmer everything together for 5–6 minutes, stirring gently, until the chicken is heated through and the sauce has thickened enough to coat the back of a spoon. Taste and adjust salt if needed. Garnish with fresh coriander just before serving.