Heat the butter or oil in a wide pan or wok over a medium heat. Add the finely diced onion and cook gently for 5–6 minutes, stirring occasionally, until softened and lightly golden.

Stir in the garlic and ginger and cook for a further minute, just until fragrant. Sprinkle in the curry powder and paprika, stirring continuously for another minute to toast the spices and release their full aroma.

Transfer the onion and spice mixture to the slow cooker. Add the chicken thighs, tomato passata, coconut milk, tomato purée, sugar, salt, and cornflour. Stir well to ensure the chicken is evenly coated and everything is fully combined.

Cover and cook on high for 3 hours or low for 5 hours, until the chicken is tender and the sauce is rich, thick, and full of flavour.

Give everything a final stir before serving.