Toss the sliced beef with cornstarch until evenly coated. Let it sit while you make the sauce.

Whisk together soy sauce, brown sugar, hoisin, garlic, ginger, sesame oil, red pepper flakes, and water. Set aside.

Sear the beef in a hot skillet with oil, in batches, until browned and crisp at the edges. Transfer to a plate.

Cook the noodles a minute shy of package directions, drain, then toss in the skillet with the sauce. Add the beef and half the green onions, toss until glossy, and serve topped with remaining onions.