The UN Delegate’s Dining Room was recently featuring Welsh cuisine during Wales Week in New York… the public are welcome for the buffet lunch at the Delegate’s Dining Room … no sales tax .. .Monday-Friday …
“Proper attire is required” – no jeans, no sneakers, jacket for men …

The Delegates’ Dining Room at UN Headquarters is open for lunch Monday through Friday. Visitors are seated between 11:30 a.m. and 2:00 p.m. as tables become available. Reservations are a must, and should be made as far in advance as possible, through the Maître d’hôtel, at (212) 963-7625. The age requirement for group reservations is 12 years or over. Proper attire is required.

From “An information guide for the public about the United Nations”
Also see “VIP lunch at United Nations Delegates’ Dining Room,” from Newyorkology, May 13, 2005
United Nations Delegates Dining Room, web site, United Nations Building, First Avenue at 45th Street, 212-963-7099
Wales in New York