Heat olive oil in a large frying pan over medium heat. Place chicken and onions in frying pan; cook 20 minutes, stirring occasionally. Mix in ground walnuts, salt, pomegranate juice and cardamom. Bring to the boil. Reduce heat to low, cover and simmer for 1 1/2 hours, stirring occasionally. (If the sauce becomes too thick, stir in 60ml cup warm water.) Mix in sugar, adjust seasoning and simmer 30 minutes more.