While the chicken roasts, tear 1 pita bread into pieces and place them in a small bowl. Pour over a couple of tablespoons of chicken stock and let it soak.
In a frying pan, dry-fry the walnuts for about 3 minutes until they're golden and smell toasted. Set them aside. Heat the remaining olive oil in the same pan and cook the chopped onion and garlic until they've softened.
Transfer the soaked pita, the cooked onion and garlic mixture, and most of the toasted walnuts to a blender. Pour in the remaining chicken stock and blend until you have a rough, textured paste. Return this mixture to the pan, stir in the single cream and a squeeze of lemon juice, season to taste, and keep it warm over low heat.