1. In a large skillet, melt 2 tablespoons of butter over medium heat. Add the sliced sweet onions and a pinch of salt. Cook slowly, stirring occasionally, for 20–25 minutes until caramelized and golden brown.

2. Push the onions to the side of the skillet. Add ground beef and cook until browned, about 6–8 minutes, breaking it apart as it cooks. Stir onions and beef together.

3. Add minced garlic, thyme, Worcestershire sauce, and beef broth to the skillet. Stir to combine and let simmer for 5–7 minutes, until slightly reduced and thickened.

4. Reduce heat to low. Stir in shredded Gruyère or Swiss cheese until melted into the mixture.

5. Butter the brioche buns and toast them on a skillet or griddle until golden brown.

6. Spoon the beef-onion mixture onto the bottom halves of the buns. Top with the other half and garnish with parsley if desired. Serve immediately.