Interested in Language
The newly-opened restaurant is selling the widest variety of dishes that I had / have seen.
Is the hyphen optional? And should it be "have' or 'had"?
Don't use hyphens with -ly words as the first word. The hyphen is not options -it's incorrect. (According to most style guides.)
Unless you have gone to a restaurant with even more dishes than this one since the time you visited, use "have."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.