Where to go to get a great steak in the middle of the day?
The Oak Room is still great and at lunch good business casual should be fine.
Several steak houses are open at lunch. In addition to the Oak Room, Abe & Louis, Ruth Chris, and Smith & Wollensky all serve lunch. And then there are places like Palm, Davio's, etc.
The difference among the top group is personal preference. I think all do a good job but I prefer Ruth Chris.
Two outstanding steak [and seafood] restaurants in Boston's Back Bay area are the Palm in the Westin Copley and Abe & Louie's on Boylston street. The Palm serves lunch M-F. A & L is open every day for lunch. Both are noted for their beef and the feedback that I've received has been positive. Both have a clubby ambiance and a proffesional wait staff. During the week they're popular with the movers and the shakers.