Cut the beef into bite-sized cubes and season with salt and pepper. Heat the butter in a large Dutch oven or heavy-bottomed pot over medium-high heat. Brown the beef in batches on all sides. Remove the beef and set aside.
In the same pot, sauté the sliced onions until softened and golden brown.
Sprinkle the flour over the onions, stir well, and cook for about a minute to remove the raw flour taste.
Return the beef to the pot and pour in the Belgian dark ale. Stir in the mustard, brown sugar, bay leaves, and dried thyme.
Spread a generous layer of mustard on the white bread slices and place them mustard-side down on top of the beef. This will help thicken the sauce as it cooks.
Cover and simmer over low heat for 2-3 hours, or until the beef is very tender and the flavors have melded together.
Serve the Carbonnade Flamande hot with creamy mashed potatoes or crusty baguette.
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