In the bottom of a 5 or 6 quart slow cooker, add butter, thyme sprigs bay leaves, sliced onions and sugar. Turn slow cooker to low and cook for 8-10 hours.
To the onion mixture in slow cooker, add beef stock, red wine vinegar, salt and pepper. Heat on high until hot. Meanwhile place oven rack at least 6 inches under broiler. Preheat broiler. Butter one side of baguette slices. Place slices, buttered side up on baking sheet. Place baking sheet under broiler. Broil slices just until toasted.
To Serve: Ladle hot French onion soup into oven-proof soup bowls. Float one or two slices of toasted baguette slices on top of soup. Sprinkle shredded cheese over the top. Place under the broiler until the cheese is melted and bubbly.
Recipe Note: 16 cups equals roughly 5 lbs of onions.