Preheat oven to 140°C. Place the beef and flour in a large bowl. Season. Toss the beef to coat.
Heat the oil in a large flameproof roasting pan over high heat. Add the beef and cook for 2 mins each side or until the beef is golden brown. Transfer the beef to a plate.
Add the onion, combined mushroom and garlic to the pan and cook, stirring, for 5 mins or until the onion softens. Return the beef to the pan with the wine or apple juice, stock, thyme and bay leaves. Bring to a simmer. Remove from heat.
Cover the pan loosely with foil. Roast, turning the beef occasionally, for 2 hours or until the beef is falling off the bone and the sauce thickens slightly. Set aside, covered, for 5 mins to rest.
Meanwhile, to make the polenta, combine the stock and milk in a large saucepan. Bring to the boil over medium-high heat. Reduce heat to medium-low. Gradually add the polenta in a thin, steady stream, whisking constantly. Cook, stirring with a wooden spoon, for 10 mins or until the polenta thickens. Remove from heat. Add parmesan and mascarpone and stir to combine. Season.
Divide the polenta among serving bowls. Top with beef and spoon over the sauce. Serve with baby broccoli.