1. Preheat your oven to 375°F (190°C).
2. In a skillet over medium heat, warm the olive oil. Add the sliced onions and cook until they are golden brown and caramelized, about 15-20 minutes. Stir occasionally to prevent burning.
3. In a large mixing bowl, combine the ground beef, breadcrumbs, beef broth, Worcestershire sauce, eggs, salt, pepper, thyme, and half of the caramelized onions. Mix gently until all ingredients are just combined.
4. Transfer the mixture to a baking dish and shape it into a loaf. Top with the remaining caramelized onions and shredded Gruyère cheese.
5. Bake in the preheated oven for 45-50 minutes, or until the internal temperature reaches 160°F (71°C).
6. Once cooked, let the meatloaf rest for 10 minutes before slicing. This helps the juices redistribute, making each slice juicy and flavorful.

Just like rolling out dough with my abuela taught me patience, allowing the meatloaf to rest is key to a perfect slice.