Inside-Out Roquefort Cheeseburgers
2 ounces Roquefort cheese, softened
4 tablespoons unsalted butter, softened
1 1/2 pounds ground sirloin
Four 1/2-inch-thick slices of sweet onion, such as Vidalia
Four 1/2-inch-thick beefsteak tomato slices
2 tablespoons extra-virgin olive oil
Salt and freshly ground pepper
Vegetable oil, for the grill
4 kaiser rolls, split
4 ounces arugula
In a small bowl, blend the Roquefort with the butter. Scrape the butter onto plastic wrap and roll into a 4-inch cylinder. Refrigerate until firm, about 1 hour. Slice the butter into 4 disks.
Gently shape the ground sirloin into 4 thick patties. Using your thumb, make a depression in the center of each patty and fill with a disk of Roquefort butter. Fold the meat over to encase the butter completely and gently pat the burger into a plump, round patty.
Light a grill. Thread each onion slice onto a skewer. Brush the onion and tomato slices with 1 tablespoon of olive oil and season with salt and pepper. Lightly brush the grate with vegetable oil. Grill the onions over a medium-hot fire until charred and tender, about 5 minutes per side. Grill the tomato slices until charred, about 1 minute per side.
Season the burgers with salt and pepper and grill over a medium-hot fire until browned and cooked through, about 6 minutes per side. Transfer to a platter and let stand for 2 minutes. Brush the cut sides of the rolls with the remaining 1 tablespoon of olive oil and grill until lightly browned, about 10 seconds. Set the burgers on the rolls, top with the onion, tomato and arugula and serve.
MAKE AHEAD The Roquefort butter can be refrigerated for 2 days or frozen for 1 month. The uncooked burgers can be refrigerated for 6 hours