In a small bowl, whisk together the tamari, brown sugar, water, and sesame oil until the sugar is dissolved. Set the sauce aside.
Toss the sliced flank steak with the cornstarch in a bowl until every piece is evenly coated.
Heat 2 tablespoons of the vegetable oil in a large wok or skillet over high heat until smoking. Add the beef in a single layer, working in batches, and sear for 1 to 2 minutes per side until crispy and browned. Remove the beef to a plate.
Add the remaining 1 tablespoon of oil to the wok. Add the garlic and ginger and stir for 30 seconds until fragrant.
Pour the sauce into the wok and bring to a rapid simmer. Cook for 1 to 2 minutes until the sauce begins to thicken slightly.
Return all the beef to the wok along with the green onion pieces. Toss everything together for about 1 minute until the sauce is thick, sticky, and coats every piece of beef.
Serve immediately over steamed rice while the beef is still crispy on the edges.