Heat the sunflower oil in a large, heavy-based casserole dish over a moderate heat. Season the beef, then sear in batches until golden brown in colour all over. Remove from the dish, then add the onion, carrot, celery bay leaf and thyme sprigs and sweat the vegetables for 6-8 minutes, stirring occasionally until just softened.
Sprinkle the beef with the flour to coat, then add back to the dish. Increase the heat and deglaze the dish with the beer, stirring the base well to loosen any debris.
Pour in the stock and bring the mixture to the boil. Stir in the tomato puree, then cover the dish and reduce to a simmer. Cook gently for 2 hours, stirring occasionally.
Remove from the heat after 2 hours and adjust the seasoning if necessary. Make sure you discard the bay leaf and thyme sprigs before serving.