In a slow cooker, toss the onions with the butter and sugar. Cook, covered, on high heat, until the edges begin to brown, about 90 minutes.
In a large skillet over medium-high heat, warm the oil until shimmering, about 1 minute. Add the beef and brown, stirring occasionally, 5-7 minutes. Transfer the beef to the slow cooker. Add the sherry to the skillet and scrape up the browned bits. Add the pan juices to the slow cooker along with the broth, Worcertershire sauce, salt, thyme, bay leaf, and pepper to taste. Simmer, covered for 2-1/2 hours on high heat or 7 hours on low.
Uncover and simmer for 1 hour more, until thick. Remove the bay leaf.
When ready to serve, turn on the broiler. Ladle the soup into flameproof bowls and divide the cheese over the tops. Place the bowls on a baking sheet and broil until the cheese is melted and bubbling, about 3 minutes. Serve hot.