Actually, the filet mignon was not available in the main dinning room. (A sirloin was.) And it was worth it for the quiet table for two.
It's not a regular menu item on seven-day cruises, so you have to request it a day in advance. Nonetheless, Princess's dining staff will prepare anything upon request, so long as they have the ingredients available and they receive the request sufficiently in advance to allow time for the preparation.
BTW, filet mignon often does appear on the menu one night, accompanies by a few shrimp ("Surf 'n' Turf") on cruises longer than a week.