Melt butter in a large skillet over medium heat. Add onions and sugar; cook 18 to 22 minutes until deeply golden.
Season beef with salt and pepper; arrange in a 6-quart slow cooker.
Spread onions over beef; add broth, Worcestershire, thyme, and bay leaf.
Cover; cook on low for 7 1/2 to 8 hours until beef pulls apart easily.
Discard bay leaf. Shred beef into large pieces; stir in balsamic vinegar.
Top baguette slices with Gruyere. Broil 2 to 3 minutes until cheese bubbles and edges brown.
Ladle beef and onion broth into bowls; top each with one cheese toast.