In a large skillet, melt the olive oil and butter over medium heat. Add the sliced onions and sauté until golden brown and caramelized, about 20–25 minutes. Add minced garlic in the last minute.

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

Preheat your oven to 350°F (175°C). Transfer the meat mixture to a baking dish or loaf pan and shape it into a loaf.

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

Combine the remaining caramelized onions with beef broth in the skillet; simmer until thickened, about 5–7 minutes.

Remove the meatloaf from the oven, top with Gruyère cheese, and broil for 3–5 minutes until melted and bubbly.

Let the meatloaf rest for 5–10 minutes before slicing. Serve warm with the onion sauce.