You can book it at City Hall as well as via +33 1 60 30 40 50. We found, though, that they are a bit short of PCs in City Hall so it took a while. DLP Info has the Walts Menu at http://www.dlp.info/News/2012/04/the-di ... ms-dinner/
Given a choice of Walts or Silver Spur for the same price we went for the former. It was tasty and well presented, but the portions were quite small. As an aside, the state of the washrooms in Walts was pretty bad, the sinks were blocked in both the Mens & Womens. Indeed every loo we went in throughout the two days left a bit to be desired.