In a small saucepan combine soy sauce, sugar, water, mirin, ginger, garlic, and red pepper flakes. Bring to a boil; reduce heat and simmer. 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. Remove from heat, stir in honey, 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.
To Make the Hibachi Steak and Wasabi Potatoes:
Remove steak from the refrigerator and let rest at room temperature for 30 minutes prior to grilling.
Meanwhile, 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. Transfer to a medium bowl. Add sesame oil, wasabi paste, and salt and toss to coat. Thread the potatoes onto skewers lengthwise.
Prepare a charcoal or gas grill for direct grilling over medium-high heat (about 375 degrees). Season the steak on both sides with salt and pepper. Oil the grill rack. Grill steak directly over medium-high heat, turning 2 to 3 times, until well browned, about 6 to 10 minutes total depending on the 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.
If using Bamboo skewers, they must be soaked for at least 30 minutes prior to grilling.For desired doneness, cook beef until thermometer registers: