Heat olive oil in a large skillet over medium heat. Place chicken and onions in skillet, and cook 20 minutes, stirring occasionally. Mix in pureed walnuts, salt, pomegranate juice, and cardamom. Bring to a boil. Reduce heat to low, cover, and simmer for 1 1/2 hours, stirring occasionally. (If the sauce becomes too thick, stir in 1/4 cup warm water.) Mix in sugar, adjust seasoning, and simmer 30 minutes more.