Make a marinade by combining sake, mirin, soy sauce, sugar, and gochujang in a large bowl.
Slice your beef if you are not using a presliced cut and add to the marinade.
Fill a large bowl with hot tap water and soak the udon.
Prepare your vegetables of choice.
In a nonstick pan or wok, preheat a small amount of oil over medium-high heat. Using a pair of tongs or chopsticks, remove the beef from the marinade and add to the pan. Set aside the marinade and stir fry your beef for 1-2 minutes, then add vegetables and fry until the vegetables are very lightly cooked.
Drain and add the udon and reserved marinade. Toss well and bring the marinade to a boil, then fry for 1-2 more minutes, or until udon is warmed through.
Enjoy!
