Belgian Beef Stew (Carbonnade)

Season the beef cubes with some salt and freshly ground black pepper. Heat up the olive oil in a 6-quart dutch oven over high heat. Sear the beef cubes, in batches to avoid overcrowding, in the oil for about 3 to 4 minutes, turning to brown all sides. Plate the beef, then set aside.Add the onions to the same pot, and cook for about 6 to 7 minutes, until the onions are soft. Season with a little salt and freshly ground black pepper. Add the thyme, bay leaves, beer, beef stock, ½ teaspoon salt, and ¼ teaspoon freshly ground black pepper. Bring to a boil, then return the beef to the pot. Bring to a boil again, then reduce to a simmer over medium-low or low heat. Simmer, uncovered, for 1 hour and 30 minutes, or until the beef is tender.Remove and discard the thyme sprigs and bay leaves. Stir in the red wine vinegar and light brown sugar. Whisk together the melted butter and flour, then stir into the pot. Simmer for another 5 minutes, uncovered. Ladle the beef stew into serving bowls/plates.Spread dijon mustard over the baguette slices, and serve with the beef stew. Alternatively or additionally, you can serve this beef stew with Belgian-style fries and/or cooked egg noodles.