Now that you have your ingredients ready, let’s dive into making these scrumptious meatballs.

1. Preheat your oven to 375°F (190°C).
2. In a large bowl, combine the ground beef, breadcrumbs, egg, salt, and pepper. Mix until just combined, being careful not to overwork the meat.
3. Form the mixture into 1-inch meatballs and set aside.
4. In a large skillet, heat the olive oil over medium heat. Add the meatballs in batches and brown them on all sides, about 3-4 minutes per batch. Remove and set aside.
5. In the same skillet, add the butter and sliced onions. Cook over medium heat, stirring occasionally, until the onions are caramelized and golden brown, about 15-20 minutes.
6. Pour in the beef broth and Worcestershire sauce, scraping up any browned bits from the bottom of the skillet. Let the mixture simmer for 5 minutes to reduce slightly.
7. Return the meatballs to the skillet, spooning the onion mixture over them. Sprinkle the shredded cheese on top.
8. Transfer the skillet to the preheated oven and bake for 15 minutes, or until the cheese is bubbly and golden.
9. Garnish with fresh thyme before serving, if desired.