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1
Heat the oven to 375 degrees F and arrange the rack in the middle.
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2
In a small bowl, mix truffle slices with short rib meat until evenly combined; set aside.
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3
Heat a medium frying pan over high heat until hot, about 4 minutes.
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4
Add foie gras to the pan and sear until browned, about 15 to 20 seconds per side.
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5
Set foie gras aside, reserving fat in the pan.
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6
Reduce heat to low, add carrot, celery, and white onion to the pan, and cook until soft, about 1 minute.
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7
Season with salt and freshly ground black pepper.
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8
Add rib mixture and reserved braising juices, and stir until meat and vegetables are well coated.
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9
Remove from heat and set aside.
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10
Season sirloin well with salt and freshly ground black pepper.
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11
Form into a tightly packed ball, then create an indentation about the size of a golf ball in the middle.
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12
Fill the indentation with 2/3 of the rib mixture and top with foie gras.
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13
Cover foie gras with remaining rib mixture, then enclose sirloin around the filling so mixture is completely encased.
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14
Flatten burger slightly to form a thick patty.
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15
Set a metal rack inside a baking sheet and place burger on top.
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16
Roast for 15 minutes, or until an instant-read thermometer inserted in the middle of the patty reads 120 degrees F. Remove from the oven and assemble sandwich.
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17
Spread 1/2 of the mayonnaise on the bottom bun, then layer it with red onion, tomato slices, frisee, and burger.
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18
Top with butter and arrange tomato confit over that.
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19
Spread remaining mayonnaise on top bun, and use to close burger.
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20
Serve on a plate with a knife and fork.
