Melt the butter in a large saucepan over a medium heat and gently fry the onions, celery and carrots until they start to soften.
Stir in the flour and cook for 2 minutes. Add the chicken stock and bring the mixture to the boil, stirring as you do so. Season with salt and pepper, then reduce the heat until the mixture is simmering and simmer for 10 minutes, until the vegetables are tender.
Add the cooked chicken and cook until heated through. Adjust the seasoning, stir in the parsley and serve.