Add the chicken to a large oven tray. Add the lemon juice, yogurt, garlic, ginger, spices, korma seasoning, salt, pepper, sugar and oil, then mix really well so everything is evenly coated. Spread into a single layer.
Scatter the chopped onion over the top in a loose layer, then dot the butter all over the onions so it melts evenly as it cooks.
Leave to marinate for at least 20 minutes if you have time. The longer the better.
Preheat the oven to 220°C fan.
Place the tray in the oven and cook for 20 minutes, allowing the onions to soften and lightly caramelise on top.
Remove from the oven, give everything a good mix, then pour in the passata, double cream and (if it's already runny maybe not stock) chicken stock. Stir until combined.
Return to the oven and cook for another 25 minutes, stirring once halfway through, until the chicken is fully cooked and the sauce has thickened.
Finish with fresh coriander and serve with basmati rice and warm naan.
