French Onion Soup Burgers with Gruyere and Balsamic Onions

In a large skillet, heat 2 tablespoons of olive oil and 2 tablespoons of butter over medium-low heat. Add sliced yellow onions and cook, stirring occasionally, for 25-30 minutes, or until deeply caramelized and golden brown. Season with salt and pepper to taste.

While the onions are caramelizing, in a small saucepan, combine the balsamic vinegar and brown sugar. Bring to a simmer over medium heat and cook for 5-7 minutes, or until slightly thickened. Remove from heat and set aside.

In a bowl, gently mix ground beef with Worcestershire sauce, garlic powder, salt, and pepper. Form into 4 equal patties.

Heat 1 tablespoon of olive oil in a grill pan or on a grill over medium-high heat. Grill burgers for 4-5 minutes per side for medium doneness, or until cooked to your liking. During the last minute of cooking, place a slice of Gruyere cheese on each burger to melt.

While the burgers are grilling, toast the brioche buns.

To assemble the burgers, spread a layer of caramelized onions on the bottom bun. Top with a cheesy burger, a generous spoonful of balsamic onions, and the top bun. Serve immediately.