Creating Mongolian Beef is straightforward if you follow these simple steps:

1. Marinate the Beef: In a bowl, combine soy sauce, brown sugar, and cornstarch. Add the sliced flank steak and mix well. Let it marinate for about 10 minutes.
2. Prepare the Pan: Heat vegetable oil in a large skillet over medium-high heat until hot.
3. Cook the Beef: Add the marinated beef to the skillet in a single layer. Cook for 2-3 minutes without stirring to allow it to sear. Then, stir-fry for an additional 2-3 minutes until browned and cooked through.
4. Add Aromatics: Add minced garlic and ginger to the skillet. Stir and cook for about 1 minute until fragrant.
5. Make the Sauce: If desired, add red pepper flakes for heat. Stir to combine everything, allowing the sauce to thicken slightly.
6. Garnish: Remove from heat and stir in sliced green onions.
7. Serve: Serve the Mongolian Beef over a bed of cooked rice, allowing the sauce to soak in.

These steps will guide you in creating this incredible dish effortlessly.