In a large bowl, combine the ground beef, breadcrumbs, egg, minced garlic, thyme, salt, and pepper. Mix gently until just combined, being careful not to overwork the meat. Form the mixture into 1-1.5 inch meatballs and set aside.

In a large skillet, heat a tablespoon of olive oil over medium heat. Once hot, add the meatballs in a single layer, browning them for about 4-5 minutes on each side until golden brown. Remove and set aside.

In the same skillet, add the sliced onions and a pinch of salt. Sauté over medium-low heat for 20-30 minutes, stirring occasionally until caramelized. Sprinkle a touch of sugar for quicker caramelization.

Pour in the beef broth, scrape the bottom of the skillet to release browned bits, and return the meatballs to the skillet. Simmer for about 5-10 minutes until heated through.

Sprinkle the Gruyère cheese generously over the meatballs and onion mixture, cover with a lid, and cook on low heat for 2-3 minutes until melted.

Serve hot with crusty bread, creamy mashed potatoes, or pasta, enjoying the comforting flavors.