Mix spinach, scallions, tamari, 2 teaspoons sesame seeds, ginger, sesame oil and garlic in a medium bowl.
Place steak on a cutting board and cover with a sheet of plastic wrap; pound with a meat mallet or heavy skillet to an even ¼-inch thickness. Spread the spinach mixture over the steak, leaving a 1-inch border. Starting at one edge of a long side, roll the steak up tightly, tucking in the filling as you go. Tie the steak with kitchen string at 2-inch intervals. Season with salt and pepper.
Heat grapeseed oil in a large cast-iron skillet over medium-high heat. Add the steak and cook, turning often, until browned on all sides, about 3 minutes total. Transfer the pan to the oven and roast until an instant-read thermometer inserted in the center of the steak reads 120°F to 125°F for medium-rare, 12 to 15 minutes.
Transfer the steak to a clean cutting board, tent with foil and let rest for 5 minutes. Cut into 8 slices and serve sprinkled with the remaining 1 teaspoon sesame seeds.
264 calories;15 g fat(4 g sat); 3 g fiber; 5 g carbohydrates; 27 g protein; 118 mcg folate; 68 mg cholesterol; 1 g sugars; 0 g added sugars; 8,387 IU vitamin A; 6 mg vitamin C; 138 mg calcium; 3 mg iron; 419 mg sodium; 564 mg potassium
Vitamin A (168% daily value), Folate (30% dv)