All ingredients ready? Let's begin!
Slice the beef against the grain. Gather and prepare all of your vegetables.
In a small mixing bowl, whisk together the soy sauce, mirin, sugar, sesame oil, and cornstarch until the sugar and cornstarch dissolve. Season the beef with 1 tablespoon of the sauce and set the rest of the sauce aside.
In a wok on high heat bring about 3 cups of water to a boil. Drop the udon noodles into the water and blanch for just 1 minute or 2 until they separate from one another. Drain the noodles in a colander and rinse under cold water. Set the noodles aside.
Reduce the heat under the wok to medium-high. Add the oil and beef to the hot wok and stir fry for about 2 minutes until the beef starts to brown.
Move the beef to one side of the pan and add the onions and garlic and cook them for about 1 minute until fragrant. Add the mushrooms and continue to cook for another minute. Add the bok choy and toss everything together, including the beef, to combine.
Place the udon noodles on top of the stir-fried vegetables and beef. Pour the remaining sauce over everything and toss again to coat evenly. Cook until the sauce reduces and everything is glossy, approximately 2 minutes. Transfer the yaki udon to a serving bowl or platter and top with the cilantro and sprinkle the sesame seeds over the top. Serve immediately.