What is the point of mustard-frying burger patties? I mean the type where mustard is placed on the hot griddle, and the patty is placed or smashed on top.
Mustard is actually mostly water (with some vinegar and mustard powder mixed in). The water inhibits browning of the patty and causes the patty to be steamed, so it does not seem like a great idea to put it below the patty.
There are other ways to add mustard flavor to the burger, like adding mustard or mustard powder to the sauce, or smearing mustard or mustard powder on the patty after it has been cooked. You could even season the patty with mustard powder before cooking it.
So, why place the patty on that mix of water, vinegar and mustard powder, which we know as mustard?
I did various tries and ate a lot of smashed cheeseburgers lately. Here is what I found:
The best option is, in the case of smashed burgers, to fry the first side without mustard, and the second side very briefly with mustard. This tastes better than frying without mustard at all, frying both sides with mustard, frying just the first side with mustard, smeering mustard on the patty after frying or adding more mustard into the sauce or elsewhere on the burger.
I cannot say why, but I can say that at least for my palate and my wife's palate mustard frying only the second side very briefly is best.