Caramelize Onions: Heat olive oil in a large skillet over medium heat. Add sliced onions and cook slowly, stirring occasionally, until soft and golden brown, about 10 minutes.
Add Garlic: Stir in minced garlic and cook for 1 minute until fragrant.
Brown Beef: Push onions to the side. Add ground beef to the skillet and break it up with a spoon. Cook until browned, about 5-7 minutes.
Season: Add Worcestershire sauce, soy sauce, thyme, salt, and pepper. Stir everything together and cook for 2-3 minutes.
Toast Buns: Lightly toast hamburger buns until golden.
Assemble: Spoon beef mixture onto bottom buns. Top each with a slice of Swiss cheese. Place under broiler for 1-2 minutes to melt cheese, or cover skillet briefly.
Garnish: Sprinkle with fresh parsley. Add top buns and serve immediately.