1. 'a banquet'is 'Gochisou'in Japanese.
2. add 'sama'makes it respectful
3. At this moment, eating is past. So you can add 'deshita'

In short, you can say, Gochisousama. 'Chisou' means runnning.'Go'is added in polite situation. the word show appreciation for someone made a effort to prepare meal. We say this word to gratitude for cook, farmer, family member who help us to living,and to animals and vagitables which give us its life.