Directions :

1. In a small saucepan combine soy sauce, sugar, water, mirin, ginger, garlic, and red pepper flakes. Bring to a boil; reduce heat and simmer.
2. Remove 2 tablespoons sauce and add to a small bowl and whisk in cornstarch. Return to the saucepan and continue simmering until the sauce thickens slightly, about 10 minutes.
3. Remove from heat and cool to room temperature while preparing the rest of the meal, or cover and refrigerate for up to one week. Bring to room temperature before using.
4. To prepare the wasabi potatoes:
5. In a medium saucepan, bring 4 quarts water and 1 tablespoon salt to a boil. Add potatoes and parboil, about 5 minutes (do not overcook). Drain and cool slightly, until you can handle the potatoes.
6. Transfer potatoes to a medium bowl. Add sesame oil, wasabi paste, and salt and toss to coat. Thread the potatoes onto skewers lengthwise.
7. To cook the steak and potatoes:
8. 20 minutes before grilling, remove steaks from refrigerator and let stand, covered, at room temperature. Meanwhile, turn all grill burners to high, cover, and heat grill until hot, about 10 to 15 minutes. Clean cooking grate.
9. Brush steak with olive oil and sprinkle liberally with kosher salt and pepper on both sides. Add steak to grill and cook until golden browned and charred on one side, about 4 to 5 minutes.
10. Flip steak and continue to grill 3 to 5 minutes for medium-rare (an internal temperature of 130 to 135 degrees F), 5 to 7 minutes for medium (140 to 145 degrees F) or 8 to 10 minutes for medium-well (150 degrees F). Transfer steak to a platter and tent loosely with aluminum foil. Let the steak rest while grilling the potatoes.
11. Grill the skewered potatoes over medium-high heat, turning 2 to 3 times, until browned and cooked through, about 10 minutes.
12. 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.