French Onion Soup Burgers

In a large skillet, melt the butter with the olive oil over medium heat. Add the onions and cook, stirring occasionally, until they are caramelized and golden brown, about 25-30 minutes. Add the beef broth to deglaze the pan, stirring until the liquid evaporates. Set aside.
In a large bowl, combine the ground beef, Worcestershire sauce, salt, black pepper, and fresh thyme. Mix gently until just combined. Form the mixture into 4 equal patties.
Preheat a grill or grill pan to medium-high heat. Cook the patties for about 4-5 minutes per side, or until they reach your desired level of doneness. During the last minute of cooking, place a slice of Gruyère cheese on each patty to melt.
In a small bowl, mix the mayonnaise and Dijon mustard until well combined. Spread this mixture on the cut sides of each brioche bun.
To assemble the burgers, place each patty on the bottom half of a bun. Top with a generous portion of caramelized onions and the top half of the bun. Garnish with fresh parsley if desired.
Prep Time: 20 minutes | Cooking Time: 30 minutes | Total Time: 50 minutes