Sear the Beef (Optional but Recommended):
Season beef with salt and pepper, then lightly dust with flour. In a hot skillet with oil or butter, sear beef in batches until browned. Transfer to the slow cooker.
Caramelize the Onions:
In the same skillet, add a bit more oil if needed. Cook onions over medium heat for 10–12 minutes until golden and soft. Add garlic and brown sugar; cook 1 more minute. Deglaze the pan with a splash of beer.
Add Ingredients to Slow Cooker:
Add caramelized onions, garlic, Dijon mustard, vinegar, beer, beef broth, thyme, and bay leaves to the slow cooker with the beef. Stir to combine.
Slow Cook:
Cover and cook on LOW for 8 hours or HIGH for 4–5 hours, until beef is fork-tender.
Finish & Serve:
Discard bay leaves. Taste and adjust seasoning. Serve hot, garnished with parsley if desired.