Step 1: Prepare the Beef

1. Pat the beef cubes dry with paper towels and season generously with salt and pepper. Toss the beef in flour to coat evenly.

Step 2: Sear the Beef

1. Heat the oil in a large Dutch oven over medium-high heat.
2. Sear the beef in batches until browned on all sides, about 2-3 minutes per side. Remove the beef and set it aside.

Step 3: Caramelize the Onions

1. Lower the heat to medium and add the onions to the pot. Sauté for 10-15 minutes, stirring occasionally, until the onions are deeply caramelized.
2. Add the garlic and cook for 1-2 minutes until fragrant.

Step 4: Build the Sauce

1. Stir in the brown sugar, Dijon mustard, and red wine vinegar. Cook for 2 minutes to develop the flavors.
2. Deglaze the pot with the beer, scraping up any browned bits from the bottom.

Step 5: Simmer the Stew

1. Return the beef to the pot and add the beef stock, thyme, and bay leaf. If using mustard-covered bread, place the slices on top of the stew.
2. Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 2 hours, or until the beef is fork-tender. Stir occasionally.

Step 6: Adjust and Serve

1. Remove the thyme, bay leaf, and bread (if used). Adjust the seasoning with additional salt and pepper.
2. Serve hot, garnished with parsley, alongside mashed potatoes, buttered noodles, or crusty bread.