Heat 1 tablespoon olive oil in a large skillet over medium heat.

Add the thinly sliced onion. Cook 10–12 minutes, stirring often, until the onions are deeply golden and caramelized. Reduce heat if they brown too quickly.

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

Add the ground beef. Break it up with a spatula and cook until fully browned.

Drain any excess fat from the skillet. Return the pan to medium-low heat.

Stir in Worcestershire sauce, soy sauce, dried thyme, and salt and pepper to taste. Simmer 4–5 minutes for the flavors to meld.

While the meat simmers, toast the hamburger buns briefly under a broiler or in a toaster until golden.

Spoon the beef-and-onion mixture onto the bottom halves of the buns. Top evenly with shredded Swiss cheese. Close the sandwiches and let the residual heat melt the cheese, or pop them under the broiler for 1 minute to fully melt.

Garnish with chopped parsley if desired and serve immediately.