In a large skillet, melt butter and olive oil over medium heat. Toss in the thinly sliced onions, cooking for 20-25 minutes. Stir occasionally until they turn deep golden and caramelized, adding minced garlic for the last 1-2 minutes.

In a large bowl, combine ground beef, eggs, breadcrumbs, Worcestershire sauce, dried thyme, salt, and pepper. Gently fold in half of the caramelized onions.

Preheat your oven to 350°F. Shape the meatloaf mixture into a loaf and place it in a pan. Bake for 45-50 minutes, or until it reaches an internal temperature of 160°F.

While the meatloaf bakes, use the same skillet to combine the remaining caramelized onions and beef broth. Let it simmer for about 5-7 minutes.

Once the meatloaf is fully baked, remove it from the oven and generously top it with grated Gruyère cheese. Broil for an additional 3-5 minutes.

Allow the meatloaf to cool for 5-10 minutes before slicing. Serve warm with the savory French onion sauce drizzled over each slice.