Marinate the Chicken: In a medium bowl, combine the chicken pieces with all the marinade ingredients: plain yoghurt, lemon juice, turmeric, ground cumin, chilli powder, and salt. Mix thoroughly to ensure every piece is well-coated. Cover and set aside for at least 15 minutes at room temperature, or up to an hour in the fridge.

Sauté the Aromatics: Heat the oil in a large, heavy-bottomed pan or Dutch oven over a medium heat. Add the cumin seeds and cook for about 30 seconds, until they sizzle and become fragrant. Add the finely chopped onions and a pinch of salt. Cook, stirring occasionally, for 8-10 minutes until the onions are soft, translucent, and just starting to turn golden at the edges.

Build the Spice Base: Add the minced garlic, grated ginger, and chopped green chilli (if using) to the pan. Cook for another 2 minutes, stirring constantly, until you can smell their strong aroma. Be careful not to let the garlic burn.

Bloom the Ground Spices: Turn the heat down to medium-low. Add the ground coriander, turmeric powder, and 1 tsp of garam masala to the pan. I find that stirring these spices into the oil and cooking them for one minute really awakens their flavours. This step is crucial for developing depth in the final curry.

Create the Sauce: Pour in the tin of chopped tomatoes. Use the juice to scrape any flavourful bits from the bottom of the pan. Add the optional teaspoon of sugar now if your tomatoes are particularly sharp. Bring to a gentle simmer and cook for 5-7 minutes, until the sauce has thickened slightly and the oil begins to separate at the edges.

Cook the Chicken: Add the marinated chicken and any remaining marinade to the pan. Stir well to coat the chicken in the tomato sauce. Increase the heat to medium, bring back to a simmer, then reduce the heat to low, cover the pan with a lid, and cook for 15 minutes, stirring once or twice.

Finish and Serve: Uncover the pan. The chicken should be cooked through. Pour in the coconut milk and stir gently to combine. Let it heat through for 2-3 minutes, but do not let it boil vigorously. Taste and adjust the salt if needed. Stir in a final pinch of garam masala and the fresh, chopped coriander. Let it rest for 5 minutes off the heat before serving.