In a large pot or Dutch oven, heat the butter over medium-high heat.
Add the beef chunks and brown them on all sides. Remove the beef from the pot and set aside.
In the same pot, add the finely chopped onions and cook until they are golden brown and caramelized, about 15 minutes.
Add the minced garlic and cook for another 1-2 minutes until fragrant.
Sprinkle the flour over the onions and garlic, and stir to combine. Cook for another 2-3 minutes to cook off the raw flour taste.
Add the brown sugar, Dijon mustard, and red wine vinegar to the pot. Stir well to combine and deglaze the pot, scraping up any browned bits from the bottom.
Return the browned beef to the pot.
Pour in the beef broth and dark Belgian beer.
Add the bay leaves, dried thyme, and dried marjoram. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low.
Spread the Dijon mustard on the slices of bread and place them on top of the stew (optional). They will dissolve and thicken the stew as it cooks.
Cover the pot and let the stew simmer gently for 2-3 hours, or until the beef is tender and the flavors have melded together. Stir occasionally and add more broth or beer if needed.
Season the stew with salt and pepper to taste.
Remove the bay leaves before serving.
Garnish with freshly chopped parsley.
Enjoy your meal!