Mustard... wow. I never thought to do a pattern on a monosteel like that, but it makes sense. Nifty!
One question though: Wouldn't a simple mustard etch result in a pattern that will dilute and get messed up rather quickly over time as other foods interact with the metal and patina it? It's basically "just" a stain so working stains will overwrite it rather quickly, no?
Yep, there's no way it will stay this way if used.
It's is purely decorative, but if the owner doesn't use some kind of abrasive the original pattern will stay on, making a nice foundation for all the new patina that will form.