Blanch the scallions. Trim the ends off each scallion, then cut into pieces about 3-inch long. Bring a saucepan of water to a boil, then add 1 tsp Kosher salt. Add the scallions and cook for 30-45 seconds, or until just beginning to wilt. Submerge the scallions in an ice bath, then drain and transfer to a paper towel-lined plate.
Prepare the steak. Place steak in a freezer for 20 minutes to make it easier to slice. When ready, trim off excess fat (it's okay to leave some on, just remove large pieces!), then slice against the grain into pieces that are about ½-inch thick.
Pound the steak. Place three pieces of sliced beef on a piece of plastic wrap leaving a few inches in between each. Top with another piece of plastic wrap, then use a meat mallet to pound into ⅛-inch thick slices. Set aside, then repeat with remaining steak.
Assemble the maki rolls. Working on a cutting board, overlap three pieces of steak to form a rectangle. Season lightly with Kosher salt, then place 3-4 pieces of crisp scallion along the short end. Roll the steak, then secure with kitchen twine or toothpicks. Repeat with remaining steak and scallions.
Coat with rice flour, optional. Place ¼ cup rice flour in a medium bowl. Toss each maki roll very lightly with rice flour, dusting off any excess.
Cook the negimaki. Heat a large cast iron or heavy bottom skillet over medium-high heat. Add 2 Tbsp neutral oil. When hot, add negimaki beef rolls and cook for 2-3 minutes per side, about 8-9 minutes total, until seared and crispy on the outside, but medium-rare at the center. Right as you turn the beef for the last time, pour in all of the teriyaki sauce. It will bubble, then simmer and thicken further.
Slice, then serve. Allow the meat to rest for 5-10 minutes, then remove twine and slice into 1-inch thick pieces. Enjoy with sesame seeds and extra sauce drizzled on top!