Add all other curry ingredients (except yoghurt), then re-add onions into a jug and blitz till smooth. Then add back into pan, re-add chicken and slowly add yoghurt (take off heat to stir yoghurt through), then simmer on low for an hour or so. Squeeze half the lemon in. Then serve. Garnish with lemon and coriander with few drops of yoghurt. Serve with steamed basmati rice.