Place your 1 lb (450g) of flank steak in the freezer for about 15 minutes to firm it up. Using a sharp knife, cut the meat across the grain into very thin strips.

In a small bowl, whisk together the dark soy sauce, regular soy sauce, oyster sauce, brown sugar, and beef broth. In a separate small bowl, stir the cornstarch with cold water to create a slurry.

Bring a pot of water to a boil, add the udon noodles, and cook for 1-2 minutes until loosened. Drain and rinse under cold water.

Heat a wok or large skillet over medium-high heat. Add the neutral oil, then the beef in a single layer. Sear for 1-2 minutes until a dark crust forms. Flip, add the minced garlic, and stir-fry for 30 seconds.

Pour the sauce mixture into the pan with the beef. Let it bubble for 1 minute, then stir in the cornstarch slurry. Cook until it transforms into a glossy dark brown sticky sauce.

Add the drained udon noodles to the pan and toss vigorously to coat. Remove from heat and generously garnish with chopped fresh green onions and toasted white sesame seeds.