Thinly slice the beef against the grain. In a bowl, marinade the beef from coconut aminos to arrowroot starch. Stir-well. Set aside in the fridge for 15-20 minutes while you prepare the vegetables.
Rinse and wash yu choy. Set aside to drain well. Dice and separate the stems from the leafy parts. For large/thick stems, you can either halve them or dice them into smaller rounds.
Chop garlic, ginger, and scallions. Separate white and green scallion parts.
In a small bowl combine and stir-well arrowroot and water to make a slurry, if using.
In a well-heated large cast iron or stainless steel skillet, add 1 tbsp avocado oil. Pan sear the beef over medium-high heat in one layer without disturbing for about 2 minutes or until the beef is in golden brown color. Use a spatula with a firm tip to flip and cook for about 30 seconds. Off heat, set the beef aside in one bowl along with the pan juices.
While the skillet is still hot, add 3 tbsp avocado oil. Fry the garlic, ginger, and white scallion parts with a pinch of salt over medium-heat for about 10 seconds. Stir-fry and toss often. Take care not to burn the aromatics.
Turn the heat up to medium-high, add yu choy stems. Season with 2 pinches of salt and saute for about 2 minutes. Add the leafy parts, season with another pinch of salt. Keep stir-frying for 1 minute and if the skillet feels dry, add 1 more tbsp oil.
Add the beef along with the pan juices back to the skillet. Add oyster sauce. Quickly give the slurry another stir then pour it over the beef and yu choy. Keep tossing and stir-frying for 10 seconds.
Off heat, serve over a large serving plate and garnish with green scallion parts.