French Onion Soup Burgers with Gruyere and Caramelized Onions - Quickie Recipe

Ingredients

• 2 tablespoons unsalted butter
• 2 large yellow onions, thinly sliced
• 1 teaspoon fresh thyme leaves
• 1/2 teaspoon kosher salt
• 1/4 teaspoon black pepper
• 1/3 cup (80 ml) low-sodium beef broth, plus more as needed
• 1 1/2 lb (680 g) ground beef, preferably 80/20
• 1 tablespoon Worcestershire sauce
• 1 teaspoon kosher salt
• 1/2 teaspoon black pepper
• 1 tablespoon neutral oil, for the skillet
• 6 oz (170 g) Gruyere cheese, grated or thinly sliced
• 4 brioche hamburger buns, split
• 1 tablespoon unsalted butter, softened, for buns
• 1/3 cup (75 g) mayonnaise
• 1 tablespoon Dijon mustard
• 1 tablespoon whole-grain mustard