Bring a large pot of water to a boil. Cook your chosen noodles according to the package directions until al dente. Drain and rinse with cold water to prevent sticking and stop the cooking process. Set aside.

While the noodles cook, prepare the sauce. In a small bowl, whisk together the soy sauce, brown sugar, sesame oil, minced garlic, and gochujang until the sugar is dissolved.

Heat vegetable oil in a large skillet or wok over high heat. Add the thinly sliced beef in a single layer, working in batches if necessary to avoid overcrowding. Cook for 1-2 minutes per side until nicely browned but still tender. Remove the beef from the skillet and set aside.

Reduce the heat to medium. Add the cooked noodles and the prepared sauce to the skillet. Toss continuously for 2-3 minutes until the noodles are well-coated and have absorbed some of the sauce.

Return the cooked beef to the skillet with the noodles. Toss everything together for another minute until the beef is heated through. Serve immediately, garnished with toasted sesame seeds and sliced green onions.