1. 1. Prepare the Beef: In a medium bowl, combine the thinly sliced beef with 2 tablespoons of soy sauce and 1 tablespoon of cornstarch. Toss well to coat the beef evenly. The cornstarch helps to tenderize the beef and create a slight crispness when cooked, while also thickening the sauce later. Let it sit for at least 5 minutes while you prepare other ingredients.

2. 2. Cook the Noodles: While the beef marinates, cook your lo mein or stir-fry noodles according to package directions. Most stir-fry noodles cook very quickly, often in just 3-5 minutes. Drain them well and toss with 1/2 tablespoon of sesame oil to prevent sticking. Set aside.

3. 3. Saute Aromatics: Heat the vegetable oil and remaining 1/2 tablespoon of sesame oil in a large wok or a large non-stick skillet over high heat until shimmering. Add the sliced onion and stir-fry for 2-3 minutes until it starts to soften and become fragrant. Then, add the minced garlic and grated ginger, stirring for another 30 seconds until aromatic. Be careful not to burn the garlic.

4. 4. Cook the Beef: Push the aromatics to one side of the wok. Add the marinated beef in a single layer to the empty side of the wok and cook for 2-3 minutes without stirring, allowing it to sear and brown. Then, stir-fry the beef with the aromatics for another 2-3 minutes until it’s mostly cooked through and no longer pink. Remove the beef and aromatics from the wok and set aside.

5. 5. Build the Sauce: In the same wok, reduce the heat to medium-high. Add the hoisin sauce, rice vinegar, brown sugar, and red pepper flakes (if using). Stir constantly for 1-2 minutes until the sauce thickens slightly and becomes glossy. It should coat the back of a spoon.

6. 6. Combine Everything: Return the cooked beef and aromatics to the wok. Add the cooked noodles to the wok as well. Toss everything together vigorously with tongs, ensuring the noodles and beef are thoroughly coated in the savory sauce. Continue to stir-fry for 2-3 minutes, allowing the noodles to absorb the flavors and heat through.

7. 7. Serve and Garnish: Transfer the beef and noodle stir fry to serving plates. Garnish generously with sliced green onions and a sprinkle of sesame seeds. Serve immediately with extra soy sauce on the side if desired.