Cut the flank steak in half lengthwise and set in a rectangular dish. Combine the soy, sugar, ginger, salt, garlic, and chile flakes and pour over steak. Cover and chill, turning occasionally, for 6 hours. Do your best not to pierce the meat.
Preheat oven to lowest setting, around 200° F. Lift meat from marinade and place on a wire rack set in a rimmed sheet pan. Bake until meat is firm and dry and no longer has a raw texture in the thickest part (cut to test), 10–12 hours. Cool meat and slice thinly across the grain to serve. Keep airtight in refrigerator for up to a week.