Slice the flank steak against the grain into thin strips, about 1/4 inch thick. Add pieces to a bowl, sprinkle with the cornstarch, and toss to coat.
Warm oil in a large pan over medium-high heat. Gently shake off any excess cornstarch and add steak pieces to the pan in a single layer. Cook for 1-2 minutes on each side, just until browned, then transfer to a plate. To get the best crispy edges, work in batches to avoid over-crowding the pan.
Once all the steak pieces are browned and removed to a plate, add garlic and ginger to the empty pan. Cook for about 30 seconds, then stir in the soy sauce, water, and brown sugar.
Bring the mixture to a boil, stirring frequently and scraping up any browned bits from the bottom of the pan. Once the sauce bubbles, return the steak to the pan. Let everything simmer for 1-2 minutes, so the sauce thickens slightly.
Add the green onions and snow peas. You can stir them in, sprinkle them on top, or serve on the side. Serve right away. Enjoy!
Notes
Many recipes for Mongolian beef call for more sugar than this. If you prefer even your savory dishes to be on the sweet side, feel free to add more, up to 1/2 cup.
The snow peas are optional. Leave them out entirely, serve them on the side, sub with broccoli, etc. I usually leave a few out of the beef mixture to serve separately for kids.