Make Ahead: The salsa and balsamic mayo can be made a day or two in advance; refrigerate in separate containers with plastic wrap directly on the surface. You may have a little salsa verde left over, which would be great on garlic bread or stirred into scrambled eggs.
Tested size: 4-8 servings
- For the salsa verde
- 1 thick slice day-old crusty white bread
- 2 tablespoons water
- 1 tablespoon capers
- 3 anchovy fillets
- 1 small clove garlic
- Leaves from 8 to 10 stems flat-leaf parsley (1 packed cup)
- 1/2 teaspoon white wine vinegar
- 5 tablespoons olive oil
- Kosher or sea salt
- For the burgers
- 1 pound very cold ground beef (preferably 92 percent lean)
- 1/2 cup packed freshly grated Parmigiano-Reggiano cheese
- Kosher or sea salt
- Freshly ground black pepper
- 1 tablespoon olive oil
- 2 tablespoons unsalted butter
- 4 hamburger buns, preferably potato
- For the balsamic mayonnaise
- 1/4 cup mayonnaise
- 2 1/2 teaspoons aged balsamic vinegar
DirectionsFor the salsa verde: Place the bread in a medium bowl. Pour the water over it and let it soak in for a minute or two, then tear the bread, letting the pieces fall into a food processor. Drain the capers and anchovy fillets.
Cut a few slices off the garlic clove and add to the food processor, along with the parsley leaves, capers, anchovies, vinegar, oil and a pinch of salt. Puree until fairly smooth, stopping to scrape down the sides of the bowl as needed. The yield is about 1 cup.
For the burgers: Place the chilled ground beef in a mixing bowl. Fold in the cheese until well incorporated. Season lightly with salt and pepper. Divide the mixture into 4 equal portions and form each into a 3/4-inch-thick patty.
Heat the oil in a large skillet over medium-high heat. Once the oil shimmers, add the burger patties and cook for 2 to 3 minutes per side, until well seared and just cooked through (medium-rare). Transfer to a cutting board to rest while you prepare the buns.
Melt the butter, then use it to spread on the inside of the hamburger buns. Toast in a toaster oven until golden brown.
For the balsamic mayonnaise: Stir together the mayonnaise and balsamic vinegar in a small bowl, until well incorporated.
To serve, spread each bottom bun with one-quarter of the balsamic mayo. Place a burger on top, then spread salsa verde over each burger and finish with the top buns. Serve warm.