Heat olive oil in a large skillet over medium heat until shimmering. Add the sliced onions with a pinch of salt and cook for 10 to 12 minutes, stirring often, until deep golden and caramelized. Do not raise the heat or the onions will burn.

Stir in the minced garlic and cook for 1 to 2 minutes until fragrant.

Add the ground beef to the skillet. Break it apart with a spatula and cook until fully browned with no pink remaining, about 5 to 7 minutes. Carefully drain any excess fat.

Stir in Worcestershire sauce, soy sauce, dried thyme, salt, and pepper. Mix well to coat all the beef evenly.

Reduce heat to medium-low and let the mixture simmer for 5 minutes. The filling should look moist but not watery.

While the filling simmers, toast the hamburger buns in a separate pan or under the broiler until lightly golden.

Spoon the beef mixture onto the bottom half of each bun. Top with shredded Swiss cheese. Optional: broil open sandwiches for 1 to 2 minutes until cheese is bubbly. Add top bun, garnish with parsley, and serve immediately.