The chef's dinner is so new I do not believe it is even being promote yet. I booked mine when I got onboard just by asking about it because I had read a press release.
Things may change by the time you sail - if you can book it onlin then definitly do that. Otherwise, book it once you get onboard, immediately, because it will fill up.
A very reasonable price - $75, most ships charge $100 to $150+