Place the flank steak in the freezer for 20 minutes to firm up — this makes slicing much easier. Once firm, slice very thin (about 1/4 inch) against the grain.

Pat the sliced beef completely dry with paper towels. This is critical for getting a good sear. Season lightly with salt and pepper, then toss in the cornstarch until every piece is evenly coated. Shake off any excess.

In a small bowl or measuring cup, whisk together the soy sauce, dark brown sugar, and water until the sugar is fully dissolved. Set aside.

Heat 1 teaspoon of vegetable oil in a large wok or heavy-bottomed skillet over medium-high heat. Add the minced garlic, grated ginger, and red pepper flakes (if using). Stir constantly for 30–45 seconds until fragrant — do not let them burn.

Pour the soy sauce mixture into the pan with the aromatics. Bring to a boil, then let it simmer for 3–4 minutes, stirring occasionally, until the sauce thickens slightly and becomes glossy. Remove from heat and set aside.

Wipe out the pan and return it to the highest heat your stove can produce. Add the remaining vegetable oil and heat until it is shimmering and just beginning to smoke.

Add the beef in a single layer in batches — do not crowd the pan. Sear for 1–2 minutes per side until deeply browned and crispy on the edges. Remove to a plate and repeat with remaining beef.

Return all the seared beef to the pan over high heat. Pour the prepared sauce over the beef and toss quickly to coat every piece. Let everything cook together for 1–2 minutes until the sauce thickens further and fully lacquers the beef.

Remove from heat and drizzle with sesame oil. Toss once more.

Serve immediately over steamed white rice, garnished generously with sliced green onions and sesame seeds.