FRENCH ONION BURGERS

2 tablespoons extra-virgin olive oil
1 large onion, halved and thinly sliced
1/2 teaspoons 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)


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 teaspoons salt, reduce the heat to medium low and cook, stirring occasionally, until deep golden, 15 more minutes.Add the chicken broth and 1 teaspoons 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. From into four 3/4-inch-thick patties and place on a broiler pan. Make an indentation in the center of each with your thumbs; season the patties with salt. Broil, turning once, 5 minutes.
3. Place the buns cut side up on a baking sheet. Sprinkle with both cheese and broil until melted, about 1 minute. Spread mustard on the buns; sandwich with lettuce, the burgers and caramelized onion. Serve with potato chips.