Toss sliced flank steak with 1 tbsp soy sauce and baking soda. Let sit at room temperature while you prep everything else. The baking soda is the secret to that velvety Chinese-restaurant texture.

Drain shirataki noodles and rinse thoroughly under cold water for 30 seconds. Boil for 2 minutes, then drain again. Dry roast in a hot dry skillet for 3-4 minutes until squeaky and most moisture evaporates. Set aside.

Stir together 3 tbsp soy sauce, oyster sauce, sesame oil, rice vinegar, and xanthan gum in a small bowl. The xanthan gum will look clumpy at first - keep stirring until smooth.

Heat 1 tbsp oil in a large skillet or wok over high heat until smoking. Add beef in a single layer and let sear undisturbed for 1 minute. Stir-fry 1-2 minutes more until just browned with some pink remaining. Transfer to a plate.

Add remaining 1 tbsp oil to the pan. Add broccoli and 2 tbsp water. Cover and steam 2 minutes until bright green and crisp-tender. Uncover and let any remaining water evaporate.

Push broccoli to the sides. Add garlic, ginger, and red pepper flakes to the center. Stir-fry 30 seconds until fragrant. Add noodles and sauce, tossing constantly for 1 minute until everything is coated and glossy.

Return beef and any accumulated juices to the pan. Toss 30 seconds just to reheat. The sauce should cling rather than pool. Serve immediately topped with scallions and sesame seeds.