You can add other veg when frying the onion and garlic stage. For example, I like mange tout. For a richer taste add 1/2 teaspoon sesame oil when marinating the beef in step 2. Black pepper can be used instead of white pepper, but the flavour is stronger.
The recipe doesn't say when to add the mushrooms so I'm guessing at the same time as the onions. Will post a review later but it sounds delicious. - 30 Oct 2017
Tastes as good as the local chinese takeaway version. - 12 Aug 2017