Heat butter and olive oil in a skillet over medium heat. Add onions, sugar, and salt. Cook until caramelized, about 25 minutes.
Transfer caramelized onions to a slow cooker. Add beef broth, white wine, Worcestershire sauce, bay leaf, and thyme.
Cook on low for 6-8 hours or on high for 3-4 hours.
Remove bay leaf before serving. Ladle soup into oven-safe bowls.
Top each bowl with a slice of baguette and shredded Gruyère cheese.
Place bowls under a broiler until cheese is melted and bubbly.