Heat olive oil in a large skillet over medium heat until shimmering. Add the thinly sliced onions and cook them slowly, stirring often, for about 10-12 minutes until they turn golden brown and caramelized.
Once the onions have reached that perfect caramelized state, stir in the minced garlic and cook it for an additional 1-2 minutes until fragrant.
Add the ground beef to the skillet with the onions and garlic. Using a spatula, break up the meat as it cooks, ensuring it browns evenly without clumping together.
After draining any excess fat from the skillet, stir in Worcestershire sauce, soy sauce, dried thyme, salt, and pepper. Allow this flavorful mixture to simmer on low heat for about 5 minutes.
While the beef mixture simmers, toast your hamburger buns until they are golden brown and slightly crispy on the outside.
Spoon generous portions of the beef mixture onto the bottom half of each toasted bun. Top with shredded Swiss cheese allowing it to melt slightly before placing on top buns; serve immediately garnished with fresh parsley if desired!