Season the flour with salt and pepper on a plate. Coat the beef with the flour.
Heat the oil in a flameproof casserole and fry the beef in batches until browned on all sides, turning it slowly in the oil. Transfer to a plate. Add the onions and celery to the casserole and fry gently for 6–8 minutes until softened.
Tip any flour left over from coating into the casserole and cook, stirring, for 1 minute. Push the vegetables to one side and return the beef to the centre of the pan. Add the bay leaves, rosemary, garlic and stock. Bring to a simmer, stirring, then cover and cook in a preheated oven, 150°C (300°F), Gas Mark 2, for 1½ hours.
Stir all the walnuts, the mustard and parsley into the casserole, re-cover and return to the oven for a further 45 minutes. Season to taste with salt and pepper and serve with steamed green beans garnished with parsley.