- 2 tablespoons extra virgin olive oil
- 1 large onion, thinly sliced
- 1/2 teaspoon sugar
- Kosher salt
- 1/2 cup low-sodium chicken broth
- 2 tablespoons Worcestershire sauce
- 1 1/2 pounds 85% lean ground beef
- 1 teaspoon onion powder
- 4 hamburger buns (I used pretzel buns)
- 2 cups shredded gruyere cheese
- 2 tablespoons grated parmesan cheese
- Dijon mustard, lettuce, sweet potato fries, for serving
Heat the olive oil in a medium 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, 10 to 15 more minutes. Add the chicken broth and 1 teaspoon of the Worcestershire sauce and simmer until the onion is tender and the liquid is almost evaporated, about 8 minutes.
Preheat the broiler. Gently mix the beef, onion powder, salt and pepper to taste, and the remaining 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. Broil, turning once, 5 to 6 minutes.
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 and top with with lettuce, the burgers and caramelized onion. Serve with sweet potato fries.