Step 1: Prepare the beef by slicing the flank steak very thinly against the grain. For easier slicing, partially freeze the steak for 20-30 minutes beforehand.
Step 2: In a medium bowl, combine the sliced beef with 2 tablespoons of cornstarch and 1/2 teaspoon of salt. Toss well to ensure every piece of beef is coated. This velvets the beef, making it incredibly tender.
Step 3: In a small bowl, whisk together the soy sauce, water, dark brown sugar, rice vinegar, minced garlic, grated fresh ginger, and red pepper flakes. Add 1 teaspoon of cornstarch to this mixture and whisk until smooth. Set aside.
Step 4: Heat 2 tablespoons of vegetable oil in a large wok or heavy-bottomed skillet over high heat until it shimmers. Add half of the beef in a single layer, ensuring not to overcrowd the pan. Sear for 1-2 minutes per side until nicely browned. Remove the seared beef to a clean plate.
Step 5: Add the remaining 1 tablespoon of vegetable oil to the wok, if needed, and repeat the searing process with the remaining beef. Once all beef is seared, set it aside.
Step 6: Pour the prepared sauce mixture into the hot wok. Bring the sauce to a simmer, stirring constantly, until it thickens to a glossy consistency, about 2-3 minutes. Make sure the sauce comes to a boil to activate the cornstarch.
Step 7: Return all of the seared beef to the wok. Toss gently to coat the beef thoroughly with the rich, thickened Mongolian beef sauce.
Step 8: Add the sliced green onions to the wok. Toss for another 30 seconds to 1 minute, just until the green onions are slightly wilted but still retain their vibrant color and a slight crunch.
Step 9: Remove the wok from the heat immediately. Serve your homemade Mongolian beef hot over steamed white rice, brown rice, or noodles. Garnish with a sprinkle of sesame seeds if desired. Enjoy your restaurant-quality meal!