Why This Recipe Works

Koji is an ancient mold that’s used to produce soy sauce, miso, fermented bean paste, and sake. Shio koji is another traditional use of this mold. It’s made by combining rice koji (cooked rice that has been inoculated with koji spores and then dried), salt, and water and letting the mixture ferment for about a week, during which it develops a sweet, fruity, slightly funky aroma. Shio koji, which can be made at home or purchased, is often used as a marinade because it is rich in protease enzymes (which break down proteins), so it can create an array of new flavors in protein-rich foods. We marinated flank steaks in shio koji to give them the nutty, savory, blue-cheese-y flavors typically found in expensive dry-aged steaks. After wiping the marinade from the steaks, we cooked them in the oven with a gentle heat to keep their interiors juicy and rosy pink and then quickly seared them on the stovetop to create a slightly charred crust.

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Before You Begin

Shio koji is available online or in the refrigerated sections of Asian markets. We prefer brands without added sugar and the chunkier, more viscous versions with the rice left in. (See “Shopping for Shio Koji” for more details.) Thoroughly trimming all of the silver skin from the steak will make it easier for the shio koji to penetrate. We like the flavor of the steak after 4 hours of marination. Marinating for longer will intensify the dry-aged flavors. To serve two people, halve all of the ingredients and sear the steaks in a 10-inch skillet. We recommend serving the steaks with Shio Koji Butter.

Instructions

Cut 1 (1½- to 1¾-pound) well trimmed flank steak in half lengthwise. Halve each piece crosswise to create 4 steaks. Place steak and 3 tablespoons shio koji in bowl and rub shio koji evenly into all sides of steak. Cover steaks and refrigerate for at least 4 hours or up to 24 hours.

Adjust oven rack to middle position and heat oven to 275 degrees. While oven heats, set wire rack in rimmed baking sheet.

With damp towel, wipe koji from steaks (it’s OK if a few grains remain on steak). Sprinkle with ½ teaspoon pepper. Set steaks on prepared rack and transfer to oven. Cook until thickest part of largest steak registers 130 to 135 degrees, 25 to 30 minutes.

Heat 1 teaspoon vegetable oil in 12-inch nonstick skillet over medium-high heat until shimmering. Add steaks and cook, pressing steaks gently into pan until browned and slightly charred in spots on both sides, 1 to 2 minutes per side. Cut steaks on bias against grain into ½-inch-thick slices. Serve, passing flake sea salt and lemon wedges, if using, separately.