Favorite early bird
Within walking distance is Perbacco, which does a great job with your preferred fare, and it should be possible to get seated then. If you're feeling more adventurous I would say walk in at Frances which is otherwise a difficult reservation to make. Great food and an easy cab ride.
Be sure to go up to the right-hand corner of this page and do a search for Union Square. You will find there have been many threads here with recommendations & comments on dining in & around Union Square. Here is a link to one:
There is a good Italian restaurant called FINO in the Andrews Hotel, which is in the 600 block of Post Street, 2 1/2 blocks west of Union Square.