
Thinly slice the beef against the grain and marinate with 1/4 teaspoon of salt, garlic, oyster sauce and freshly cracked black pepper.
Trim off the top and bottom of the onion, peel, cut in half and then thinly slice. Cut baby bok choy into shorter pieces. If the stems are large, slice in half lengthwise.
Mix together fish sauce, soy sauce, water (or beef stock) and cornstarch to make the sauce. Set aside.
Cook dried pho noodle according to package instructions but take it out 1 minute earlier than what is recommended in the instructions. Run it through cold water thoroughly, then set aside.
Put a pan over medium high heat. When it is hot, add some oil and the beef. Let the beef cooked undisturbed for about 30 seconds and then start stir-frying. When it is about 80% cooked, transfer to a plate.
Add a little more oil to the pan and then add onions and baby bok choy. Quickly stir-fry to soften them.
Add rice noodles and beef back to the pan and the sauce. Stir-fry and mix well until everything is fully cooked. Adjust seasoning to taste and sprinkle with more black pepper. Transfer to clean plates and serve immediately.