1.Preheat oven to 180°C/160°C fan.
2.Cut beef into 4cm pieces. Place beef and flour in a large bowl; season. Toss to coat beef evenly in flour, shaking off excess flour.
3.Heat 1 tablespoon of the oil in a 5 litre (20−cup) cast iron or other heavy−based casserole dish over high heat. Cook half the beef, turning, for 5 minutes or until well browned. Transfer beef to a large bowl. Repeat with another 1 tablespoon of the oil and remaining beef; transfer to same bowl.
4.Reduce heat to medium; add remaining oil, the shallots, carrot, celery, garlic and thyme. Cook, stirring, for 2 minutes or until garlic is fragrant. Add tomato paste; cook, stirring, for 3 minutes or until mixture looks almost dry. Add wine, scraping base of pan with the spoon. Bring to the boil; simmer for 2 minutes.
5.Return beef to dish; stir in balsamic vinegar and stock. Cover surface of casserole with a large piece of baking paper, ensuring mixture is completely covered; cover with a tight−fitting lid. Transfer dish to oven; bake, stirring occasionally, for 1½ hours. Remove the lid and baking paper; cook for a further 30 minutes or until beef is tender. Stir in chocolate until melted.
6.Serve half the beef casserole topped with extra thyme (see > serve it). Transfer remaining beef casserole to an airtight container; cool, then store (see > store it).
Refrigerate beef casserole for up to 3 days.
Freeze up for 3 months; thaw in the fridge and then microwave.Note