Thinly slice the beef against the grain. In a bowl, season the beef from coconut aminos to starch. Stir-well. Set aside in the fridge for 15-20 minutes while you prepare other ingredients.
Prepare garlic, ginger, scallions, and red chili peppers, if using.
Preheat a large wok or skillet over medium-low heat until it feels warm when placing your palm near it. Add 1 tbsp oil. Pan-sear the beef over medium-high heat in a single 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 the flip side for 30 seconds. Off heat, set the beef aside in one bowl along with the pan juices.
Add another tbsp of oil, and saute the garlic, ginger, white scallion parts, and chili peppers with 2 pinches of salt over medium heat for 15 seconds.
Turn the heat up to medium-high, return the beef to the pan along with the pan juice and pour in the sha cha sauce mixture. Keep stir-frying and tossing for another 10 seconds. Add in the green scallion parts and give it another quick toss. Turn off the heat.
Transfer the beef onto a large serving plate. Serve hot.
Notes
Make-ahead: Prepare the homemade sacha sauce in advance to save time when cooking the dish.
Store-bought: You can also use store-bought bullhead bbq sauce from either a local Chinese grocery store or on Amazon. (product)