Add the beef chunks, sliced onions, minced garlic, thyme, bay leaves, Worcestershire sauce, salt, and pepper to the slow cooker. Pour the beef broth over everything and stir to combine.
Cover and cook on low for 8 hours until the beef is fork-tender and the onions have softened into the broth.
About 15 minutes before serving, heat the oven broiler. Spread the baguette slices with the softened butter and place them on a baking sheet.
Broil the bread for 1 to 2 minutes until lightly toasted, then remove from the oven. Sprinkle the Gruyère cheese evenly over each slice.
Return the baking sheet to the broiler for another 1 to 2 minutes until the cheese melts and bubbles.
Remove the bay leaves from the slow cooker. Ladle the beef and broth into bowls and top each serving with two cheese toasts.