1. In a large skillet, heat olive oil over medium-low heat. Add sliced onions and cook, stirring occasionally, until caramelized, about 15 to 20 minutes.
2. Increase the heat to medium-high and add ground beef to the skillet. Cook until browned, breaking up the meat with a spatula.
3. Stir in minced garlic, Worcestershire sauce, soy sauce, dried thyme, and optional beef bouillon paste or beef broth. Season with salt and pepper.
4. Let the mixture simmer for about 5 to 7 minutes to allow flavors to meld, stirring occasionally.
5. Preheat the broiler. Toast brioche or burger buns under the broiler until golden brown.
6. Spoon the beef mixture onto the bottom halves of the buns. Top with a slice of Swiss or provolone cheese.
7. Broil briefly until the cheese melts, then add the top halves of the buns.
8. Garnish with chopped fresh parsley if desired and serve immediately.