How to Make It
In a small bowl, whisk together the tamari, mirin, red miso, sugar and sesame oil. Spread 1 teaspoon of the mixture on each side of the beef slices. Refrigerate for 1 hour. Reserve the remaining marinade.
In a saucepan of salted boiling water, cook the kale until bright green, 2 minutes. Drain and lightly squeeze out the excess water.
On a work surface, lay out a slice of beef with a long side facing you. Place 1 scallion slice across the lower edge. Top with one-eighth of the kale; some of the kale should extend beyond the short sides of the meat. Roll the meat up over the filling very tightly. Secure the roll with 2 toothpicks. Repeat with the remaining meat, scallions and kale.
Light a grill or preheat a grill pan. Brush the rolls with oil. Oil the grill grate. Grill the rolls over high heat until charred, 2 minutes. Brush the rolls with some marinade and grill for a few seconds more, until glazed.
Transfer the rolls to a work surface. Discard the toothpicks. Cut the negimaki into 1-inch lengths. Transfer to a platter, cut sides up, and drizzle with the remaining marinade. Sprinkle with sesame seeds and serve.