Chicken in white wine sauce

INGREDIENTS

• 4 skinless chicken breasts
• 1 onion
• 1 carrot
• 1 stick celery
• 1 bay leaf
• 6 black peppercorns
• 30g/1oz butter
• 30g/1oz flour
• 150ml/¼ pint white wine
• 1 free-range egg yolk
• 2-3 tbsp double cream
• salt and freshly ground black pepper
• squeeze lemon juice
• 2 tbsp chopped fresh parsley
• , to serve mashed potato