Slice the beef thinly, against the grain, and set aside.
Cut the mushrooms and bok choy and set aside.
Mince the garlic and set aside.
Cook the noodles according to the instructions on the package, then drain and set aside.
While the noodles are cooking, mix the soy sauce, minced garlic, brown sugar, ground ginger, and oyster sauce together in a small bowl until the sugar and ground ginger has dissolved. Stir in the beef broth.
While the noodles are cooking, heat the vegetable oil in a large saucepan set over medium heat. Heat until the oil is hot and shimmering.
Add the beef to the pan and sear brown on both sides.
Add the mushrooms and bok choy to the pan and cook, stirring occasionally, until the mushrooms have started to soften and the bok choy is beginning to wilt, about 3 minutes.
Add the sauce mixture and stir to combine, then bring to a simmer.
Add the noodles to the pan and stir to combine. Let cook, stirring occasionally, for another 5-8 minutes or until the sauce has thickened slightly and the mushrooms have softened all the way through.
Remove and serve hot.