Chicken Katsu Sandos

Ingredients

• 2 large scallions
• 1/4 cup mirin
• 2 tablespoons light brown sugar
• 1/2 cup soy sauce
• 1 (1-inch) piece peeled fresh ginger, thinly sliced
• 2 garlic cloves, roughly chopped
• 6 (5- to 6-ounce) boneless, skinless chicken thighs, trimmed
• 1/2 cup cold water
• 3/4 cup all-purpose flour (about 3 1/4 ounces)
• 3 large eggs, well beaten
• 2 1/2 cups panko
• Neutral cooking oil (such as peanut), for frying
• 6 cups shaved green cabbage (from 1 small head)
• 1 1/4 teaspoons kosher salt
• 1 teaspoon granulated sugar
• 2 tablespoons fresh lemon juice
• 6 tablespoons tonkatsu sauce
• 6 tablespoons drained pickled red ginger strips (beni shoga)
• 12 (3/4-inch-thick) slices Japanese milk bread, crust removed and lightly toasted, if desired
• 9 tablespoons Kewpie mayonnaise

Steps

1. Make the chicken katsu: Separate scallions into white parts and green parts. Cut white parts into 2-inch pieces; set aside. Thinly slice green parts; refrigerate in an airtight container until ready to use. Bring mirin to a simmer in a small saucepan over medium; simmer for 1 minute. Stir in brown sugar until dissolved, about 40 seconds. Stir in soy sauce, ginger, garlic, and reserved white parts of scallions; bring to a simmer. Remove from heat; let marinade steep, uncovered, for 30 minutes.
2. Meanwhile, butterfly the chicken thighs by placing 1 thigh on a cutting board, lengthwise, with smooth side down. Holding a knife at a 45-degree angle, make slices halfway through thicker parts of thigh, and fold open; repeat as needed until thickness of thigh is mostly uniform. Transfer thigh to a large ziplock plastic bag; repeat process with remaining thighs. Add 1/2 cup cold water to marinade. Pour through a fine wire-mesh strainer set over bag with chicken; discard solids. Press out as much air as possible; seal bag. Massage chicken to distribute marinade. Refrigerate at least 3 hours or up to overnight.
3. Meanwhile, make the cabbage slaw: Toss together cabbage, salt, and sugar in a large bowl. Let stand at room temperature for 20 minutes. Add lemon juice, and lightly massage using hands to soften cabbage; discard excess liquid. Cover and refrigerate until ready to use or up to 2 days. Pour off excess liquid before using.
4. Continue making the chicken katsu: Place a wire rack inside a large rimmed baking sheet; set aside. Place flour, eggs, and panko in three separate large, shallow dishes. Remove chicken from marinade; discard marinade. Working with 1 chicken thigh at a time, dredge in flour, shaking to remove excess; dip in egg, letting excess drip off. Dredge in panko, pressing to adhere. Transfer chicken to prepared baking sheet. Refrigerate, uncovered, at least 30 minutes or up to 12 hours. (Overnight is best for crispy chicken.)
5. Preheat oven to 200°F. Place a wire rack inside a large rimmed baking sheet; set aside. Pour oil to a depth of 3/4 inch into a large, deep skillet; heat over medium to 325°F. Working in batches, fry chicken, turning occasionally, until golden brown and an instant-read thermometer inserted into thickest portion of chicken registers at least 165°F, 4 to 5 minutes. Transfer chicken to prepared baking sheet. Place in oven to keep warm; repeat frying process with remaining chicken.
6. Remove chicken from oven; top each chicken piece with 1 tablespoon tonkatsu sauce, 1 tablespoon pickled red ginger strips, 1/2 cup reserved cabbage slaw, and about 1 tablespoon sliced scallion greens.
7. Drizzle 1 side of each bread slice with about 2 teaspoons mayonnaise. Top 6 bread slices with dressed chicken, and sandwich with remaining 6 bread slices, mayonnaise side down.