Heat 1 tablespoon olive oil and 2 tablespoons butter in a large skillet over medium heat. Add thinly sliced onions and cook for 20-25 minutes, stirring occasionally, until caramelized. Stir in minced garlic and cook for 1-2 minutes. Set aside.

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

Preheat oven to 350°F (175°C). Shape the meat mixture into a loaf in a baking dish or loaf pan.

Bake for 45-50 minutes, or until the internal temperature reaches 160°F (71°C).

In a skillet, add the remaining caramelized onions and 1 cup beef broth. Simmer for 5-7 minutes to reduce slightly and thicken into an onion sauce.

Remove meatloaf from the oven and top with grated Gruyère cheese. Broil for 3-5 minutes until bubbly and golden.

Let the meatloaf rest for 5-10 minutes before slicing. Serve with the French onion sauce spooned over the top.