Heat olive oil in a large skillet over medium-high heat.
Add beef cubes and brown on all sides, about 5-7 minutes. Transfer beef to the slow cooker.
In the same skillet, add sliced onions and cook over medium heat, stirring occasionally, until caramelized, about 15 minutes.
Add minced garlic to the onions and cook for 1 more minute.
Transfer the caramelized onions and garlic to the slow cooker.
Add beef broth, tomato paste, dried thyme, dried rosemary, bay leaves, salt, black pepper, Worcestershire sauce, and balsamic vinegar to the slow cooker. Stir to combine.
Cover and cook on low for 6 hours, or until beef is tender.
About 10 minutes before serving, preheat the oven broiler.
Ladle soup into oven-safe bowls.
Place one slice of whole wheat bread on top of each bowl of soup.
Sprinkle shredded Gruyère cheese evenly over the bread slices.
Place the bowls on a baking sheet and broil in the oven until the cheese is melted and bubbly, about 3-5 minutes. Watch carefully to prevent burning.
Remove from oven and let cool slightly before serving.