Place the beef strips in a bowl and add 1 tablespoon peanut butter, 1 teaspoon stevia, 1 teaspoon chili (sambal), and 1 tablespoon tamari, and use a spoon to combine it well. If you have time to leave it to marinate, that's great but if not, that's fine too.
Mix the other half of the peanut butter, stevia, chili, and tamari in a small bowl with 3 tablespoons water.
Heat oil in a large pan. When the pan is hot, add the beef and fry off for just a couple of minutes. Use tongs to toss it so both side of the beef strips are brown. The trick is not to overcook the beef, or it becomes tough. So just a very quick fry is all that's needed.
Set the beef aside in a bowl.
Add the wombok to the pan and saute for a couple of minutes until it starts to soften and wilt. Throw the choy sum in and the rest of the sauce and cook for 1 minute to wilt the choy sum.
Add the beef back into the pan, heat through for 1 minute and you're ready to serve.
Notes
Choy sum is an Asian veggie, so if you don't have it you can also use bok choy, or any other green leafy veg such as kale. The good thing about choy sum or bok choy is that they have sweet tasting leaves, whereas sometimes kale can be bitter.