For beef stir fries, I love buying beef schnitzel cuts in the supermarket because they're already prepared by the butcher to be thin and tender; but prior to prepping I always tenderise the meat even more. If you're fancy and resourceful I bet you have meat tenderiser at home, but if you're like me you can use your kitchen pestle!
Prepare the beef by tenderising and cutting them to the size of your liking. I like to cut them around 2cmx2cm size. Marinate with 1:2:2 of soy sauce, oyster sauce and kecap manis, generous pinch of black pepper and cornflour. Set aside for at least 30 minutes.
Heat oil in wok, sautee chopped garlic and onion until fragrant. Add the beef with the marinade sauce in. Keep stirring until they change color. Add in your crunchy greens (my favorite is gai lan but if there's no Asian grocer around, broccoli or broccolini will suit you all good because they're similar kind of veges anyway!) and chopped capsicum. Splash with Chinese rice wine and maybe hot water if you want extra sauce on yours. Adjust taste with black pepper, salt and more sauces if you wish.
Definitely a super easy house favorite at the moment... not to mention cheap!