1/4 cup white miso (fermented soybean paste)
2 teaspoons Dijon mustard
1 teaspoon grated peeled ginger
1 teaspoon kosher salt
1 garlic clove, grated
2 tablespoons vegetable oil plus more for brushing
1 1/2 pound grass-fed beef flank steak
Flaky sea salt, such as Maldon, and freshly ground black pepper
White miso, also known as shiro miso, is available at Japanese markets, natural foods stores, and in the refrigerated Asian foods section of some supermarkets.
Whisk together miso, mustard, ginger, kosher salt, garlic, and 2 Tbsp. oil into a paste. Smear on both sides of steak. Wrap well in plastic and chill for at least 30 minutes and up to overnight.
Arrange a rack in upper third of oven and preheat to 450°. Brush a baking sheet with oil. Remove steak from marinade, shaking off excess, and place on prepared sheet. Roast, turning once, until cooked to medium, about 13 minutes. Turn on broiler; cook steak, watching closely to prevent burning, until deep brown, about 1 minute per side. Let rest for 10 minutes. Thinly slice against the grain.