For the Caramelized Onions:

• 3 large yellow onions: Yellow onions are the classic choice for French onion soup due to their balanced sweetness and savory depth when caramelized. They soften beautifully and develop a rich, complex flavor that forms the foundation of this recipe. Make sure they are truly large to yield enough caramelized onions for the burgers. If you prefer a slightly sweeter onion, you could use Vidalia onions, but yellow onions are traditional and work exceptionally well.
• 3 tablespoons unsalted butter: Butter is crucial for caramelizing onions. It adds richness, flavor, and helps the onions soften and brown evenly without burning. Unsalted butter allows you to control the saltiness of the dish, especially since we’ll be adding beef broth later. You can use salted butter if you prefer, but be mindful of adjusting the salt levels in the recipe accordingly.
• 1 tablespoon olive oil: Adding olive oil to the butter helps to raise the smoke point and prevent the butter from burning during the long caramelization process. It also contributes a subtle fruity note to the onions. Extra virgin olive oil is a good choice for its flavor, but regular olive oil will also work perfectly well.
• 1 teaspoon granulated sugar: A touch of sugar helps to speed up the caramelization process and enhances the natural sweetness of the onions. It also contributes to that beautiful deep golden-brown color. Don’t worry, the burgers won’t taste sweet; the sugar simply helps develop the savory caramelization.
• 1 teaspoon dried thyme: Thyme is a classic herb in French onion soup, imparting a warm, earthy, and slightly peppery flavor. Dried thyme is convenient and works wonderfully in this recipe. If you prefer fresh thyme, you can use about 1 tablespoon of fresh thyme leaves, but dried thyme’s concentrated flavor is ideal for caramelizing with the onions.
• 1/2 teaspoon salt: Salt is essential for drawing out moisture from the onions and seasoning them as they caramelize. It also helps to enhance the other flavors in the dish. Use kosher salt or sea salt for the best flavor.
• 1/4 teaspoon black pepper: Freshly ground black pepper adds a touch of spice and complexity to the caramelized onions. It complements the sweetness of the onions and the earthiness of the thyme.

For the French Onion Soup Broth:

• 1 cup beef broth: Beef broth is the soul of French onion soup, providing a rich, savory, and deeply umami base. Use a good quality beef broth or stock for the best flavor. Low sodium broth is recommended so you can control the salt level. If you’re feeling ambitious, homemade beef broth will elevate the flavor even further.
• 1/2 cup dry red wine (such as Cabernet Sauvignon or Merlot): Red wine adds depth, acidity, and complexity to the French onion soup flavor. A dry red wine like Cabernet Sauvignon or Merlot is ideal, as it complements the beef broth and caramelized onions without being overly sweet. If you prefer not to use wine, you can substitute an additional 1/2 cup of beef broth, but the wine really enhances the authentic French onion soup taste.
• 1 tablespoon balsamic vinegar: Balsamic vinegar adds a touch of sweetness and acidity that balances the richness of the beef broth and caramelized onions. It also contributes a depth of flavor that mimics the slow cooking process of traditional French onion soup. A good quality balsamic vinegar will have a richer, more complex flavor.

For the Burgers:

• 2 pounds ground beef (80/20 blend): Using an 80/20 blend of ground beef ensures juicy and flavorful burgers. The fat content keeps the burgers moist during cooking and contributes to their overall richness. Look for ground chuck or a blend specifically labeled for burgers. Avoid using leaner ground beef, as it can result in dry burgers.
• 1 tablespoon Worcestershire sauce: Worcestershire sauce adds a savory, umami depth to the burger patties, enhancing their beefy flavor. It also contains vinegar, which tenderizes the meat and adds a subtle tang.
• 1 teaspoon garlic powder: Garlic powder provides a savory, aromatic base flavor to the burger patties. It’s a convenient way to add garlic flavor evenly throughout the meat. You can substitute fresh minced garlic if you prefer, but garlic powder is easy to incorporate and distributes flavor well.
• 1 teaspoon onion powder: Onion powder complements the garlic powder and enhances the overall savory flavor of the burger patties. It also adds a subtle sweetness that balances the saltiness.
• 1/2 teaspoon salt: Salt is crucial for seasoning the burger patties and enhancing their flavor. Use kosher salt or sea salt for the best flavor.
• 1/4 teaspoon black pepper: Freshly ground black pepper adds a touch of spice and complexity to the burger patties.

For Topping and Serving:

• 6 slices Gruyère cheese: Gruyère cheese is the quintessential cheese for French onion soup. It melts beautifully, has a nutty, slightly sweet flavor, and develops a lovely golden-brown crust when broiled. It’s the perfect cheese to complement the caramelized onions and beefy burger. If Gruyère is unavailable, you can substitute Swiss cheese or a good quality provolone, but Gruyère truly captures the authentic flavor.
• 6 brioche buns: Brioche buns are soft, buttery, and slightly sweet, making them an excellent choice for gourmet burgers. They hold up well to juicy patties and toppings and provide a delightful textural contrast. If you can’t find brioche buns, potato buns or pretzel buns are also good alternatives.
• Fresh parsley, chopped (optional, for garnish): Fresh parsley adds a touch of freshness and color to the finished burgers. It provides a bright, herbal note that cuts through the richness of the burger and toppings. Chopped parsley is a simple yet elegant garnish.