Cut the flank steak into 1/4 inch thick strips against the grain to ensure tenderness.

In a shallow bowl, combine 1.5 tablespoons soy sauce, 1 tablespoon cornstarch, 1 teaspoon sesame oil, and 2 tablespoons vegetable oil; add the beef strips and toss to coat, then let rest for 5 to 10 minutes at room temperature.

Pour 1/2 cup cornstarch into another shallow bowl; working in batches, dredge the marinated beef strips in the cornstarch, shaking off any excess.

Heat 3 cups of vegetable oil in a large skillet or wok over medium-high heat until it reaches 350°F (about 3 to 4 minutes).

Fry the coated beef strips in two batches for 2 to 3 minutes each, stirring occasionally, until golden brown and crispy; maintain oil temperature between batches.

Transfer the fried beef to a paper towel-lined plate and drain excess oil; let rest while you prepare the sauce.

In a medium saucepan over medium heat, sauté the minced garlic and ginger for about 30 seconds until fragrant, then add the red pepper flakes and cook for another 15 seconds.

Stir in the hoisin sauce, remaining 1.5 tablespoons soy sauce, rice vinegar, honey, and water; cook for 1 to 2 minutes until the sauce is smooth and glossy.

Add the fried beef to the sauce and gently toss to coat each piece evenly, working quickly to preserve the crispy exterior.

Transfer the Beijing beef to a serving platter, garnish with sliced scallions and sesame seeds, and serve immediately to maintain the crispy-chewy contrast.