Step 1

Gochujang is available at Korean markets and online.

Step 2

Purée scallions, cilantro, vinegar, gochujang, ginger, soy sauce, sesame oil, pepper, sugar, 1 garlic clove, and ¼ cup vegetable oil in a blender until smooth. Set aside ¼ cup marinade and chill.

Step 3

Place steak and remaining marinade in a large resealable plastic bag, turn to coat, and chill at least 2 hours.

Step 4

Remove steak from marinade, scraping off as much marinade as possible. Heat 1 Tbsp. vegetable oil in a large skillet over medium-high heat. Cook steak until browned on all sides, 8–10 minutes for medium-rare (the thinner steaks will cook more quickly). Transfer to a cutting board and let rest at least 5 minutes before slicing.

Step 5

Heat remaining 1 Tbsp. vegetable oil in a medium skillet over medium-high heat. Add bok choy, cut side down, and cook until golden brown, about 2 minutes. Turn, add remaining garlic clove, and cook, tossing occasionally, until fragrant, about 1 minute. Add ¼ cup water and cook until bok choy is tender, about 2 minutes longer.

Step 6

Top steak with chives and serve with bok choy and reserved marinade alongside.

Step 7

DO AHEAD: Steak can be marinated 1 day ahead. Keep chilled.