Brown the ground beef in a large skillet over medium heat, breaking it up as it cooks. Drain excess fat and transfer the beef to the slow cooker.
Add the sliced onions and minced garlic to the same skillet. Cook for 8 to 10 minutes, stirring often, until the onions soften and start to brown.
Transfer the onions and garlic to the slow cooker. Add the beef broth, thyme, bay leaves, Worcestershire sauce, salt, and pepper.
Stir everything together, cover, and cook on low for 6 to 7 hours.
Remove the bay leaves before serving. Ladle the stew into bowls and top each with a toasted baguette slice and a sprinkle of Gruyere cheese.