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1
Whisk the ginger, garlic, 1 tablespoon oil, soy sauce, mirin, salt, and pepper in a small bowl. Place the skirt steak in a resealable plastic bag and pour the marinade over. Turn to coat and let sit at room temperature 30 minutes.
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2
Remove the skirt steak from the marinade. Heat 1 tablespoon oil in a large skillet over medium-high heat. Add the steak and sear to desired doneness, about 3 minutes per side for medium rare. Let steak rest on a cutting board.
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3
Wipe out skillet and heat remaining 1 tablespoon oil. Sear the bok choy cut side down over medium-high heat until browned, 2 to 3 minutes. Flip the bok choy over, add 1/4 cup water, cover, and cook 2 minutes more. Remove bok choy and roughly chop.
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4
Using a mandoline, carefully slice the sweet potato lengthwise into 1/8-inch-thick planks. Select 4 planks from the center of the sweet potato that are similar in size. Meanwhile, fill a large Dutch oven or cast-iron skillet with oil to a depth of 1 1/2 inches and heat carefully to 350u00b0F. Sift the flour, cornstarch, and 1/2 teaspoon salt into a large bowl. Gently incorporate the club soda into the flour mixture; do not overmix. Coat 2 slices sweet potato in the batter and fry, turning occasionally, until golden brown, about 5 to 6 minutes. Repeat with remaining 2 slices sweet potato. Sprinkle with remaining 1/4 teaspoon salt and drain on a cooling rack placed over a sheet tray.
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5
Assemble the sandwich: Mix together the miso, mayonnaise, and lemon juice in a small bowl. Thinly slice the skirt steak against the grain. Place 1 fried sweet potato slice on a plate and layer with half the bok choy, half the skirt steak and 1 tablespoon of miso mayo. Top with another sweet potato slice to form a sandwich. Repeat with remaining sweet potato slices to make a second sandwich.
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6
The leftover miso mayo can be refrigerated for up to a week.
