Serves: Serves 4
- 2 tablespoons extra-virgin olive oil, plus more as needed
- 1½ pounds red onions, thinly sliced (about 4 cups)
- 2 tablespoons balsamic vinegar
- 1 teaspoon coarse salt
- 1⁄8 teaspoon freshly ground black pepper
- 1½ pounds ground beef
- 3 ounces firm blue-veined cheese, such as Stilton, crumbled
- 1 tablespoon finely chopped fresh Italian parsley
Heat a medium skillet, preferably cast-iron, over medium heat. Add the oil and heat until hot enough to sizzle an onion ring. Add the onions, adjust the heat to medium-low, and sauté, stirring, for 2 minutes, or until evenly coated with the oil. Cover and cook for 3 minutes, or until wilted. Uncover and cook, stirring occasionally and adjusting the heat between medium and medium-low, for about 20 minutes, or until the onions are a dark golden brown. Remove the skillet from the heat. Transfer the onions to a large bowl. Add the vinegar, salt, and pepper to the onions and set aside. Rinse and dry the pan.
Divide the ground beef into eight 3-ounce portions, and shape each portion into a patty about 4 inches in diameter. Divide 2 ounces (about ½ cup crumbled) of the cheese evenly among 4 of the patties, sprinkling it on the surface and lightly pressing it into the meat. Top with the remaining 4 patties, to make 4 cheese-stuffed burgers. Press around the edge of each burger to seal the top and bottom patties together, to prevent the cheese from oozing out during cooking. Brush both sides of the burgers lightly with olive oil.
Heat the skillet over medium heat until hot enough for a drop of water to sizzle on contact. Place the burgers in the pan, increase the heat to high, and cook, flipping them at the midpoint, for 4 minutes per side for medium-rare and 5 minutes per side for well done. Transfer the burgers to a platter.
Add the onions to the hot skillet and reheat briefly over low heat for about 1 minute, or until hot. Spoon the onions over the burgers. Sprinkle with the remaining 1 ounce of cheese and the parsley, and serve.