Preheat the oven to 400°F.
In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, chopped onion, parsley, garlic, egg, salt, pepper, and thyme. Mix until well combined.
Shape the mixture into meatballs, about 1 inch in diameter, and place them on a baking sheet lined with parchment paper.
In a skillet, heat the olive oil over medium heat. Add the meatballs and cook until browned on all sides, about 5-7 minutes. Transfer the meatballs back to the baking sheet.
In the same skillet, add the beef broth and Worcestershire sauce, scraping up any browned bits from the bottom. Bring to a simmer, then pour the broth mixture over the meatballs.
Sprinkle the shredded Swiss cheese on top of the meatballs. Bake in the preheated oven for 15-20 minutes, or until the meatballs are cooked through and the cheese is melted and bubbly.
Let the meatballs cool for a few minutes before serving. Enjoy!