Slice flank steak against the grain into 1/4-inch thick strips on cutting board using sharp knife. Pat dry with paper towels.

In a mixing bowl, toss beef strips with 1/4 cup cornstarch, salt, and pepper until evenly coated.

In another mixing bowl, whisk together soy sauce, 1/2 cup water, brown sugar, hoisin sauce, rice vinegar, sesame oil, minced garlic, and grated ginger. Set aside.

In a small bowl, mix 1 tablespoon cornstarch with 2 tablespoons water to create a slurry. Set aside.

Heat vegetable oil in grill or cast iron skillet over high heat until smoking. Working in batches, add beef in a single layer and cook for 2-3 minutes per side using tongs until crispy and browned. Transfer to a plate.

Reduce heat to medium. Pour sauce mixture into the same skillet, scraping up any brown bits from the bottom with a wooden spoon.

Bring sauce to a simmer and cook for 2-3 minutes, stirring with whisk. Add cornstarch slurry and stir constantly for 1 minute until sauce thickens and becomes glossy.

Return beef to skillet along with green onions. Toss everything together for 1-2 minutes until beef is coated and green onions soften slightly.

Remove from heat. Serve immediately over hot rice, garnished with sesame seeds and red pepper flakes if desired.