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1
Set a rack over a rimmed baking sheet.
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2
In a bowl, toss the beef chuck with 2 teaspoons of salt to evenly coat.
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3
Spread in a single layer on half of the rack.
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4
Add the short rib to the bowl and toss with 1 teaspoon of salt to evenly coat.
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5
Spread in a single layer alongside the chuck.
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6
Refrigerate the meat uncovered for 8 hours.
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7
Store the meat grinder attachmentincluding the blade and coarse grinding platein the freezer.
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8
In a nonstick skillet, heat 2 tablespoons of the oil.
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9
Add the onions and cook over moderate heat, stirring frequently, until deep golden, 25 minutes.
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10
Season with salt and keep warm.
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11
Working over a large bowl, gently pass the short rib through the grinder.
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12
Gently press the beef chuck through the grinder, gradually adding the ground short rib as you go so they are ground together.
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13
Gently shape the ground beef into ten 1-inch-thick patties, packing them just enough so they hold together.
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14
Preheat the oven to 425.
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15
Arrange the English muffins on 2 baking sheets.
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16
Set a cheese slice on each top half.
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17
Toast until the muffins are crusty and the cheese is melted, about 5 minutes.
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18
Spread some caper mayonnaise on the bottom halves.
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19
Preheat a cast-iron griddle.
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20
Add 2 tablespoons of the canola oil and cook half of the burgers over moderately high heat until browned, 2 to 3 minutes.
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21
Flip the burgers and top with half of the caramelized onions, lightly pressing them into the burger.
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22
Cook until browned and pink within, about 2 minutes for medium-rare.
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23
Repeat with the remaining 2 tablespoons of canola oil, burgers and caramelized onions.
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24
Set the onion burgers on the muffins and serve.
