In a large skillet over medium heat, add the olive oil. Once hot, add the sliced onions. Cook, stirring occasionally, for about 10-15 minutes until the onions are golden brown and caramelized.

Add the minced garlic to the skillet and cook for an additional 1-2 minutes until fragrant.

Push the onion mixture to one side of the skillet, then add the ground beef. Season with salt and pepper. Cook until the beef is browned, breaking it up with a spatula as it cooks.

Once the beef is thoroughly cooked, mix it with the onion mixture. Add in the Worcestershire sauce, soy sauce, thyme, and beef broth. Stir well to combine all ingredients.

Reduce the heat to low, cover, and let it simmer for about 5-10 minutes so the flavors meld together.

Toast your burger buns or slider rolls if desired. Spoon the beef mixture onto each bun.

Top each serving with shredded Swiss cheese, and place under a broiler for about 2 minutes, or until the cheese is melted and bubbly.

Remove from the oven, garnish with fresh parsley, and serve hot.