Heat olive oil in a large skillet over medium heat.
Add the sliced onions, salt, 1/4 teaspoon black pepper, and sugar to the skillet.
Cook the onions, stirring occasionally, for about 20 minutes until deeply caramelized and golden brown.
Push the onions to the side of the skillet and add the ground beef to the center.
Cook the beef, breaking it apart with a spatula, until browned and cooked through, about 6-8 minutes.
Add the minced garlic, dried thyme, and dried rosemary to the beef and onions. Stir to combine and cook for 1 minute until fragrant.
Pour the beef broth and Worcestershire sauce into the skillet and stir to combine. Let it simmer for 3-4 minutes to meld the flavors and reduce slightly.
Season with the remaining 1/4 teaspoon black pepper and adjust salt to taste.
Reduce heat to low and lay the Swiss cheese slices evenly over the beef and onion mixture.
Cover the skillet with a lid and cook for 2-3 minutes until the cheese is melted.
Sprinkle chopped fresh parsley over the top before serving.