Add oil, onions, thyme, bay leaf and maple syrup to bottom of slow cooker. Cover and cook on HIGH for 8 hours. Stirring once or twice if you can.
Remove bay leaf, add beef broth, vinegar, salt and pepper; cook, covered on high for 30 minutes.
Preheat broiler to high.
Divide soup between 10-12 ovenproof bowls on a baking sheet. Top each bowl with two slices of baguette and equal amounts of cheese. Broil 2-3 minutes until cheese melts and begins to brown.