Pat steak dry and lightly score each side against the grain. Salt and pepper the steak and then grill over high heat until slightly charred and internal temperature is medium (about 125-130 degrees). Remove from grill, plate and tent the meat with foil. Let rest at least 10 minutes.
Brush olive oil on scallions and salt and pepper them. Grill whole scallions until slightly charred. Slice charred scallions into 2-inch pieces.
Meanwhile, whisk together the mirin, sesame oil, soy sauce, brown sugar, vinegar, and sriracha.
Slice flank steak against the grain very thinly. Spread scallions over the meat then drizzle over some of the dipping sauce. Save some sauce to serve separately on the side. Top with sesame seeds if using.