1poundhigh-quality beef, thinly sliced and preferably either flank steak or ribeye cuts(if you're preparing a kosher version of this meal, please use kosher-certified meat)
2tbsppotato starch or cornstarch
2tbspavocado oil or another neutral oil
1/2tspsalt
1/4tspfreshly ground black pepper
1/2cupscallions(sliced into 2-inch pieces)
1/3cupred onion(sliced very thin)
1cupsliced fresh mushrooms(shiitake or button are the best varieties to use here)
Cook the noodles according to package instructions. Drain and toss with 2 tablespoons of sesame oil to prevent sticking. Set aside.
Toss the beef slices with potato starch, salt, and pepper. Let it sit for 10 minutes while you prep the vegetables.
In a small bowl, mix the soy sauce, mirin, rice vinegar, extra 1 tbsp of sesame oil, garlic, ginger, chili flakes (if using) and brown sugar. Whisk until well combined. Set aside.
Heat 1 tablespoon of oil in a large frying pan or wok. When it's hot, toss your thinly-sliced beef in, cooking until browned on all sides.
Toss your noodles in to mix with the beef and the little bits of beef in the pan.
Turn the heat off, add your umami sauce. Serve immediately.