- 3 tablespoons butter
- 1 tablespoon EVOO (extra-virgin olive oil)
- 2 onions, thinly sliced
- Kosher salt and pepper
- 1/2 teaspoon ground thyme
- 1 bay leaf
- 1/2 cup beef stock
- 1 tablespoon light brown sugar
- 1 tablespoon good-quality balsamic vinegar
- 1 1/2 pounds ground beef chuck
- 2 cloves garlic, grated or finely chopped
- 2 tablespoons Worcestershire sauce
- A handful of flat-leaf parsley, finely chopped
- EVOO (extra-virgin olive oil), for drizzling
- 1/2 pound Cambozola blue cheese, thinly sliced
- 4 brioche rolls, split and lightly toasted
- Arugula leaves, coarsely chopped
Make the onions: Heat the butter and EVOO in a large skillet over medium heat. Add the onions, season with salt and pepper, and add the thyme and bay leaf. Cook, stiffing occasionally, until the onions are caramelized, about 30 minutes. Add the stock, brown sugar, and vinegar and stir until the sugar is dissolved. Keep warm over low heat; remove bay leaf before serving.
Make the burgers: In a large bowl, combine the beef, garlic, Worcestershire sauce, and parsley; season with salt and pepper and mix thoroughly. Score the mixture into 4 equal portions and form them into patties slightly thinner at the center than at the edges for even cooking and to ensure a flat surface (burgers plump as they cook). Drizzle the patties with EVOO.
Heat a large nonstick skillet over medium-high heat. Cook the burgers 10 minutes, flipping once, for medium (adjust the cooking time for rarer or more well-done burgers). Top the burgers with Cambozola for the last minute or two of cooking, tenting the skillet with aluminum foil, if you like, to help melt the cheese.
Place the burgers on the roll bottoms, mound with caramelized onions, and top with a few leaves of arugula. Set the roll tops in place.