Pat the sliced beef dry with paper towels. Season lightly with black pepper.
Whisk soy sauce, 1/2 cup water, brown sugar, mirin, rice vinegar, garlic, ginger, sesame oil, and red pepper flakes in a bowl.
Stir cornstarch with cold water in a small cup until smooth to make a slurry.
Heat vegetable oil in a large skillet over medium-high heat until shimmering.
Sear the beef in two batches, 1–2 minutes per side, just until browned. Transfer to a plate.
Pour the sauce mixture into the skillet. Bring to a lively simmer, scraping up browned bits.
Whisk in the cornstarch slurry. Simmer, stirring, until the sauce turns glossy and thick, 1–2 minutes.
Return the beef and any juices to the pan. Toss to coat and warm through, about 1 minute.
Fold in most of the green onions and sesame seeds. Reserve a pinch for garnish and serve.