Step 1: Prepare the Beef

1. Pat the beef dry with paper towels, then season with salt and black pepper.
2. Sprinkle the flour over the beef cubes and toss to coat evenly. This helps the meat brown properly and slightly thickens the sauce.
3. Heat 1 tablespoon of oil in a Dutch oven over medium-high heat.
4. Add the beef in batches, browning each side for about 3–4 minutes per batch. Avoid overcrowding the pot. Transfer browned beef to a plate.

Step 2: Caramelize the Onions

1. Reduce heat to medium and add 2 tablespoons of butter to the same pot.
2. Add the sliced onions and cook for 15–20 minutes, stirring occasionally, until they become soft and golden brown.
3. Stir in the minced garlic and cook for another 1–2 minutes until fragrant.
4. Sprinkle in brown sugar and apple cider vinegar, stirring well to deglaze the pan.

Step 3: Build the Sauce

1. Stir in Dijon mustard and Worcestershire sauce.
2. Pour in Belgian beer, scraping the bottom of the pot to lift any browned bits.
3. Add beef broth, thyme, and bay leaf, then return the browned beef to the pot.

Step 4: Simmer the Stew

1. If using the traditional mustard-spread bread, place it on top of the stew. It will dissolve and thicken the sauce as it cooks.
2. Cover and simmer on low heat for 2.5–3 hours until the beef is fork-tender. Stir occasionally.

Step 5: Serve and Enjoy

1. Remove the bay leaf and thyme sprigs.
2. Adjust seasoning with more salt and pepper if needed.
3. Garnish with fresh parsley and serve hot with fries, mashed potatoes, or bread.