Remove steak from the refrigerator and let rest at room temperature for 30 minutes prior to grilling.
Meanwhile, bring 4 quarts water and 1 T. salt to a boil in a medium saucepan. Add potatoes and parboil for 5 minutes (do not overcook). Drain and cool slightly, until you can handle the potatoes. Transfer to a medium bowl. Add sesame oil, wasabi paste, and salt and toss to coat. Thread the potatoes on to skewers lengthwise.
Season the steak with on both sides with salt and pepper. Prepare a charcoal or gas grill for direct grilling over medium-high heat (about 375°F). Oil the grill rack. Grill directly over medium-high heat, turning 2 to 3 times, until well browned, about 6 to 10 minutes total depending on desired doneness. Check the temperature with an internal thermometer (see notes for temperatures). Remove to a platter and tent with foil while grilling the potatoes.
Grill the skewered potatoes over medium-high heat, turning 2 to 3 times, until browned and cooked through, about 10 minutes. Remove to a platter.
Transfer the steak to a cutting board and slice against the grain. Serve with the teriyaki sauce and grilled potatoes, garnishing with scallions and toasted sesame seeds.