Chicken soup

Name: Chicken soup
Ingredients: 55g/2oz butter; 2 onions, peeled and sliced; 2 sticks celery, finely chopped; 2 carrots, peeled and finely diced; 25g/2oz plain flour; 1.2 litres/2 pints chicken stock; 450g/1lb cooked chicken, skinned and shredded; 1 tbsp freshly chopped parsley; Salt and freshly ground black pepper