I love this deceptively simple dish. If you have all the ingredients, you can have a plate of delicious noodles on the table within 15 to 20 minutes, prep included.
Technique tip: Although sesame oil is a fat and you would assume it should be used to start the stir-fry, I want you to treat it like a sauce. Sesame oil has incredible aroma and flavor but burns at a low temp. Add it to a sauce instead and use a high-temp oil like canola or peanut for cooking.
Swap option: There really isn't such a thing as a "lo mein" noodle, so don't try to find it on the shelf. You want to buy an egg noodle or pasta that's relatively thin and has some tooth. Some common names will be lo mein, chow mein, egg noodles or pancit noodles. Most markets have Japanese yaki soba noodles in the cold case, and those would work perfectly. Spaghetti or fettuccine cooked al dente and rinsed in cold water and drained in a colander will also make a great lo mein. The traditional difference between lo mein and chow mein is that lo mein is a soft noodle with some gravy and chow mein is a crispy fried noodle tossed with or smothered in sauce. This has become very convoluted over the 200 years Chinese food has existed in America, with regional evolutions.
To make the sauce, stir together the oyster sauce, sesame oil, soy sauce, chicken stock and cornstarch in a small bowl and reserve.
Heat the pan to high and add the oil. Once you see wisps of white smoke, add the garlic and ginger and cook until light brown and fragrant, about 20 seconds. Stir in the chicken and cook until medium, about 1 minute. Stir in the noodles and bok choy and cook until the bok choy starts to soften and turn bright green, about 1 minute. Stir in the sauce; allow the sauce to coat all the ingredients and start to simmer, about 1 more minute. Cook until the chicken is cooked through (about 1 more minute) and sauce starts to bubble into a glaze. Top with scallions and serve hot.