Whisk soy sauce, water, brown sugar, garlic, ginger, sesame oil, and red pepper flakes; set aside.
Pat steak dry, then toss with cornstarch until evenly coated; shake off any excess.
Heat 2 tablespoons oil in a large skillet or wok over medium-high heat.
Sear half the beef until browned, about 2 minutes; transfer to a plate.
Add remaining oil and cook the rest of the beef the same way.
Pour off extra fat, leaving about 1 tablespoon in the pan.
Stir in the sauce; simmer, stirring, until slightly thickened, 2 to 3 minutes.
Return beef and accumulated juices; toss until coated and glossy, 1 to 2 minutes.
Fold in green onions; cook 30 seconds, then serve over rice.
Notes
For extra veggies, toss in steamed broccoli or snap peas with the green onions. Prefer it spicier? Add a pinch more red pepper flakes or a dash of chili oil. Leftovers keep up to 4 days refrigerated; reheat in a skillet with a splash of water to loosen the sauce.This recipe is an original creation inspired by classic Easy Mongolian Beef Recipe flavors. All ingredient ratios and instructions are independently developed.