Soy Sauce–Marinated Grilled Flank Steak and Scallions

Prepare a grill for medium-high heat; lightly oil grate. Pat steak dry and place on a rimmed baking sheet. Lightly score both sides of steak with the tip of a paring knife, making shallow parallel cuts (no deeper than an 1/8") about 1" apart and running across the grain of the meat (this will prevent the steak from curling as it cooks). Season generously all over with salt and pepper; let sit at room temperature at least 30 minutes and up to 1 hour.
Meanwhile, toss scallions on another baking sheet with 2 Tbsp. olive oil to coat; season with salt and pepper.
Whisk mirin, soy sauce, brown sugar, vinegar, Sriracha, and sesame oil in a medium bowl until sugar is dissolved; set marinade aside.
Pat steak dry again (the salt will have drawn out more moisture) and rub with remaining 1 Tbsp. olive oil. Grill scallions, turning often, until tender and charred in spots, about 1 minute. Return to baking sheet. Grill steak, turning every 2 minutes or so, until medium-rare, 6–8 minutes. Transfer to a cutting board and let rest 10–15 minutes.
Thinly slice steak crosswise and arrange on a rimmed platter; drizzle all of the juices that have collected on the cutting board over. Arrange grilled scallions on top of the steak, then pour reserved marinade over. Let sit at least 15 minutes and up to 1 hour before serving. To serve, sprinkle with sesame seeds as desired.