Slice the steak thinly against the grain to prepare it. Put it in a basin and lightly coat it with cornflour, salt, and black pepper.
Preparing the sauce Mix soy sauce, brown sugar, sesame oil, water, and cornflour in a small bowl until well combined.
Prepare the noodles: Follow the directions on the package to prepare the noodles. After draining, set away.
Sear the beef. In a big skillet or wok, heat the olive oil over high heat. Work in batches if needed, adding the beef in a single layer. Cook until browned and well cooked, 2 to 3 minutes per side. Take it out and put it away.
Add the broccoli to the same wok and stir-fry it for two to three minutes. Cook the ginger and garlic for a further 30 seconds, or until fragrant.
Put the noodles in: Add the noodles to the wok along with the cooked beef.
Pour in the sauce: Cover everything with the Mongolian sauce. For two to three minutes, toss constantly until the sauce thickens, bubbles, and covers the beef, noodles, and broccoli in a glossy layer.
Add red pepper flakes, sesame seeds, and green onions to finish. Toss again.
Serve: While hot, shiny, and steaming, serve right away.