Heat butter and olive oil: In a large skillet, heat 2 tablespoons butter and 1 tablespoon olive oil over medium heat.

Cook the onions: Add 2 large sliced onions and cook, stirring occasionally, until they become soft and golden brown, about 15-20 minutes.

Add sugar: Sprinkle 1 teaspoon sugar over the onions and continue to cook for another 5 minutes until they are deeply caramelized.

Simmer with beef broth: Pour in 1 cup beef broth and simmer until the liquid is mostly absorbed, about 10 minutes. Set aside.

Mix ingredients: In a mixing bowl, combine 1 pound ground beef, 1 teaspoon salt, ½ teaspoon black pepper, and 1 tablespoon Worcestershire sauce. Mix until just combined.

Form patties: Divide the mixture into 4 equal portions and shape them into patties.

Heat the skillet or grill: Preheat a skillet or grill over medium-high heat.

Cook the patties: Cook the burger patties for about 4-5 minutes on each side, or until they reach your desired level of doneness.

Melt the cheese: Place a slice of Swiss cheese on each patty and let it melt.

Toast the buns: Ensure 4 brioche buns are toasted and ready.

Layer the ingredients: Place the burger patties on the bottom halves of the toasted brioche buns. Top each patty with a generous amount of the caramelized onions. Garnish with chopped fresh parsley.

Complete the burgers: Place the top halves of the buns on the burgers.

Serve immediately: Pair with your favorite sides like fries or a fresh salad for a complete meal.