Ingredients

• 2 tablespoons extra-virgin olive oil
• 1 large onion, halved and thinly sliced
• 1/2 teaspoon sugar
• Kosher salt
• 1/2 cup low-sodium chicken broth
• 2 tablespoons plus 1 teaspoon Worcestershire sauce
• 1 1/2 pounds ground beef
• 1 teaspoon onion powder
• 4 brioche or hamburger buns, split
• 2 cups shredded gruyere cheese (about 6 ounces)
• 2 tablespoons grated parmesan cheese
• Dijon mustard, lettuce and potato chips, for serving (optional)

Instructions

1. Heat the olive oil in a large skillet over medium-high heat. Add the onion and cook, stirring, until it starts to brown, 3 to 4 minutes. Add the sugar and 1/2 teaspoon salt, reduce the heat to medium low and cook, stirring occasionally, until deep golden, 15 more minutes. Add the chicken broth and 1 teaspoon Worcestershire sauce and simmer until the onion is tender and the liquid is almost evaporated, 8 minutes.
2. Preheat the broiler. Mix the beef, onion powder and the remaining 2 tablespoons Worcestershire sauce in a bowl with your hands. Form into four 3/4-inch-thick patties and place on a broiler pan. Make an indentation in the center of each with your thumb; season the patties with salt. Broil, turning once, 5 minutes.
3. Place the buns cut-side up on a baking sheet. Sprinkle with both cheeses and broil until melted, about 1 minute. Spread mustard on the buns; sandwich with lettuce, the burgers and caramelized onion. Serve with potato chips.
4. Photograph by Christopher Testani