In a large skillet over medium heat, melt the butter with the olive oil.
Add the sliced onions, salt, pepper, and sugar. Cook, stirring frequently, until onions are deeply caramelized, about 25 minutes.
Add the minced garlic and cook for 1 more minute until fragrant.
Transfer the caramelized onions and garlic to the slow cooker.
Add the beef broth, white wine, Worcestershire sauce, bay leaf, and fresh thyme sprigs to the slow cooker. Stir to combine.
Add the beef chuck pieces to the slow cooker, submerging them in the liquid.
Cover and cook on low for 8 hours until beef is tender and flavors meld.
About 30 minutes before serving, preheat the oven broiler.
Arrange the baguette slices on a baking sheet and toast under the broiler until golden, about 2 minutes per side.
Remove the bay leaf and thyme sprigs from the soup and discard.
Ladle soup into oven-safe bowls, placing 2 toasted baguette slices on top of each bowl of soup.
Sprinkle about 1/3 cup shredded Gruyère cheese over the bread slices.
Place the bowls on a baking sheet and broil until the cheese is melted and bubbly, about 3-5 minutes. Watch carefully to prevent burning.
Carefully remove the bowls from the oven and serve immediately.