Remove steak from refrigerator 30 minutes before cooking. Pat dry with paper towels and season lightly with salt and pepper.

In a small saucepan, combine soy sauce, mirin, rice vinegar mixture, brown sugar, garlic, and ginger. Bring to a simmer over medium heat, stirring to dissolve sugar. Once simmering, add the cornstarch slurry and whisk continuously until the sauce thickens enough to coat the back of a spoon (about 2-3 minutes). Remove from heat and set aside.

Heat oil in a cast-iron skillet over high heat until smoking. Place steak in the pan and sear for 3-4 minutes without moving. Flip and cook for another 3-4 minutes for medium-rare (adjust time for desired doneness).

Lower heat to medium and pour half the teriyaki sauce over the steak. Flip the steak to coat both sides, allowing the sauce to caramelize slightly. Remove steak to a cutting board and let rest for 5-10 minutes.

Cut steak against the grain into 1/2-inch slices. Arrange on a plate, drizzle with additional teriyaki sauce, and garnish with sliced green onions and sesame seeds.