In a bowl, whisk together hoisin sauce, soy sauce, brown sugar, and water until well combined. Set aside.
Season sliced flank steak with salt and black pepper, then toss with cornstarch until evenly coated.
Heat 4 tablespoons of oil in a wok or heavy-bottomed pan over high heat. Sear the steak in batches until browned on both sides.
Transfer to a plate and set aside.
Reduce heat to medium-low. Add remaining oil to the pan.
Stir-fry dried red chilis for 30 seconds, then add grated ginger and pressed garlic. Cook for another 30 seconds until fragrant.
Return seared beef to the pan. Add sliced green onions and prepared sauce.
Toss everything together for about 30 seconds until heated through and well coated.
Top with roasted white sesame, serve and enjoy!