I've been pondering this subject for a few years now. hahaha Phnom Penh - Butter Beef
And this is totally wild speculation on my part.
It's been a while since I've had butter beef, but from what I recall, the dish doesn't actually arrive to the table hot. It's like room temp/lukewarm.
My theory is that the beef is all sliced, plated and fridged ahead of time.
When order comes it, it's removed from fridge.
They cover it with a plate and pop it in the steamer.
Not long enough to cook the meat, just to take the chill off.
It's then sauced, garnished and served.
If you look closely, it's a rather wide band of cooked meat, but clearly on the outside, the meat is still raw.
So I think the plate used is hot enough to cook the meat where it contacts.
This is why there are rather inconsistencies to the size and location of that grey banding.
Covering plate is totally off center
Left in steamer too long?
Too long in steamer again?