Pat the beef cubes dry with paper towels. Season them generously with salt and black pepper.

Preheat your Dutch oven or a large, heavy-bottomed pot over medium-high heat.

Add the bacon lardons to the hot pot and cook until crispy and rendered, about 5-7 minutes. Remove the bacon with a slotted spoon and set aside, leaving the rendered fat in the pot.

If needed, add 1 tablespoon of olive oil to the pot. Dredge the seasoned beef cubes lightly in flour, shaking off any excess. Working in batches to avoid overcrowding, sear the beef on all sides until deeply browned, about 3-4 minutes per batch. Remove the seared beef and set aside with the bacon.

Reduce the heat to medium. Add the sliced onions to the pot and cook, stirring occasionally, until softened and deeply caramelized, about 10-15 minutes. This step is crucial for flavor development.

Add the minced garlic and cook for another minute until fragrant.

Pour in the Belgian dark ale, scraping the bottom of the pot with a wooden spoon to deglaze and release any browned bits. Bring to a simmer and let it cook for 2-3 minutes to burn off some of the alcohol.

Return the seared beef and cooked bacon to the pot. Stir in the beef broth, Dijon mustard, brown sugar, dried thyme, and bay leaves. Ensure the beef is mostly submerged in the liquid. If not, add a little more beef broth or water.

Bring the liquid to a gentle simmer, then reduce the heat to low, cover the pot tightly, and braise for at least 2.5 to 3 hours, or until the beef is fork-tender. Check periodically to ensure it's not sticking and stir if necessary.

Once the beef is tender, remove the lid and increase the heat slightly if you desire a thicker sauce. Simmer uncovered for 15-20 minutes, or until the sauce has thickened to your preference. Remove the bay leaves before serving.

Taste and adjust seasoning with additional salt and pepper if needed. Ladle the beef carbonnade into bowls and garnish with fresh chopped parsley, if using. Serve hot with mashed potatoes, frites, or crusty bread.