Rehearsal dinner for 30-40 Near Copley
I've done two functions of about that size in the last two years in the Salon Room at Mistral. Both were extraordinary. But it's not cheap. There are different pricing options, but we paid $100/person for six passed hors d'oeuvres, one salad or appetizer, a choice of three entrees, and one dessert. (In each case, we were able to select our options from an extensive menu.) Add wine, other beverages, tax and tip, and it came to about $5,000 for 30 people. Well worth it, IMHO.
So my go to recommendation in this area is Tapeo on Newbury Street (within a couple of blocks- defiantly walking distance from the Westin). It is a tapas restaurant and they do a great job creating a festive, social environment for large parties. I have had countless company functions there and love the space, food, ambiance combination.
However, not knowing what your budget is and or the environment you are looking for I can also offer up some other recommendations including
Abe & Louies- high end steak house
Sonsie's - your party size may be too large for their wine room, but give it a shot
Atlantic Fish Company
and then of course if you're going for the wow factor (and in this case I can only say the view is a wow, the food is mediocre) there is Top of the Hub
If your party doesn't mind walking about 8 blocks or so there are also countless restaurants in the South End that would do a great job with an event of your nature.