The stars of this meal are the amazing Korean-style roasted Brussels sprouts. They are sweet, spicy, Brussel-y (in a good way), and flavored with a secret weapon: gochujang ("go-choo-tjang"). Gochujang is a fermented soybean paste mixed with chili powder that's a staple in Korean kitchens. Think of it like Sriracha meets miso. Also think of it as the only way you'll ever want to have Brussels sprouts ever again.
Spread the Brussels sprouts out onto a baking sheet and toss with the oil. Roast until beginning to brown, about 20 minutes. While the sprouts are roasting, whisk together the gochujang, orange juice, shallot and garlic. Toss the lightly browned sprouts with the gochujang mixture and continue roasting until glazed and very tender, about 20 minutes more.
While the sprouts are roasting, in a medium mixing bowl or one-gallon resealable bag, combine the soy sauce, sesame oil, vinegar, ginger, garlic, Sriracha and sugar. Add the steak to the sesame mixture and marinate for 20 minutes.
Place a large cast-iron skillet over medium heat with the oil. Cook the steak in the skillet to your desired doneness, about 4 minutes per side for medium. Let the steak rest for 5 minutes before slicing.
When the sprouts are finished, toss them with the butter and season with salt. Serve the sprouts with the sliced steak.