Prepare the beef: Slice beef thinly against the grain and toss with 2 tbsp cornstarch until coated.
Make the sauce: In a bowl, whisk together soy sauce, brown sugar, mirin, rice vinegar, garlic, ginger, and water.
Cook the beef: Heat vegetable oil in a large skillet or wok over medium-high heat. Add the beef and stir-fry for 2–3 minutes per side until browned. Remove and set aside.
Thicken the sauce: Pour the prepared sauce into the same skillet. Bring to a simmer, then add the cornstarch slurry while stirring until thickened and glossy.
Combine: Return cooked beef to the pan, toss well to coat evenly, and cook for 1–2 minutes more.
Finish and serve: Drizzle sesame oil over the top and garnish with sliced green onions and sesame seeds. Serve hot over rice or noodles.